The resource page for information for travelers to Illinois is out of date.  Following is a proposed updated page. Please make a note if there are any inaccuracies, I've tried to be as concise as possible while still being thorough. The municipalities with AW/magazine bans section in the spoiler tag I'm sure could use some input from those with local knowledge.

Thanks for your assistance.

TL:DR? Click To View Spoiler
Many are apprehensive about travelling through Illinois, so trying to be as thorough as possible. Spoiler boxes used throughout this page for additional reference material. As of 2017 there have been many changes (for the better!) in Illinois law in the last few years.

Illinois State Police no longer appear to have information on their website regarding transportation of firearms, and the only information page on the Department of Natural Resources website for hunters is out of date and contains some incorrect information.

A common misconception is that transporting a firearm through Illinois requires an Illinois Firearm Owners Identification Card (FOID). Non-residents of Illinois are not required to have a FOID nor are they able to obtain one. Illinois court precedent holds that a non-resident with a firearms permit or carry permit from their home state is the equivalent of an IL resident with a FOID for certain passages in IL law that reference a FOID, but even that is not required for a non-resident to transport a firearm through Illinois. If you are moving to Illinois, once you establish residency and get an Illinois ID or Drivers License, you have a limited time (60 days) to then also obtain a FOID.

Do not "open carry" any type of firearm in public, whether on your person or in a vehicle, regardless of whether it's loaded or unloaded. All firearms in public in Illinois must be out of public view. Discretion is not only a good idea, but legally required except in some specific and limited circumstances. Bona fide hunting (with hunting license) in a suitable location would be an example of those circumstances.

Concealed Carry (loaded handgun carry)

Illinois has concealed carry since July 2013 after passage of the Firearms Concealed Carry Act (430 ILCS 66 Firearm Concealed Carry Act). (With an assist from the 7th Circuit Court of Appeals via Moore v Madigan)

There is no full reciprocity or recognition of non-resident carry permits. Full concealed carry requires an Illinois Firearm Concealed Carry License, and only Illinois residents and those of a few other states (as of 2017 - Virginia, Arkansas, Mississippi, and Texas) are allowed to apply.  As of 2017 there is litigation in progress with the goal of opening applications to all Americans.

However, non-residents of Illinois who have carry in their home states (best with a home state carry license/permit) are allowed in-vehicle concealed carry.
Only applies to handguns due to the definition of "concealed firearm" specified in the Concealed Carry Act.

Illinois does not have proactive duty to inform, but those carrying must answer truthfully if asked by LE.

Concealed carry has been well received by LE and there have been no reports of particularly negative interactions with LE.

Other Transportation

Illinois transportation requirements are less stringent than FOPA, so if you're following FOPA you should be ok.

The most common way to lawfully transport firearms (other than handguns w/ concealed carry) is:
- unloaded, and
- in some kind of case, shipping box or other container that fully encloses the firearm.

Alternatively, firearms may be transported if "broken down into a non-functioning state", or are not immediately accessible. Regardless of the condition in which they are transported, firearms must be concealed from view (no rifle racks or the like). At a practical level a case is expected and is the best solution. IL SC decided that a vehicle center console constitutes a "case".

Magazines containing ammunition may not be inserted into the firearm or else the firearm is considered loaded. Magazines may be in the same case/container as the firearm.
Click To View Spoiler
NRA-ILA's webpage on Illinois includes:
Click To View Spoiler
You can have a loaded firearm in your own hotel room, or at a home or private property with permission. There are various exemptions for lawful target practice/instruction, hunting, and competitions for non-residents.

If you are traveling in Illinois to engage in hunting or other activities regulated by the Wildlife Code, that code more strictly defines lawful transportation and requires a case intended for firearms with a closure.

Additional information

1) "Assault weapons": Illinois does not have a statewide "assault weapon" or magazine capacity ban.

Several local jurisdictions in the greater Chicago metro area have various AW and/or magazine bans. The Illinois State Police maintains a list of municipalities with local firearms ordinances. Local firearms restrictions are frozen due to a provision in the 2013 Concealed Carry Act.
Click To View Spoiler
Something of note is that Chicago (plus reportedly Cicero and Thornton) ban laser sights on all firearms.

It is a subject of debate whether magazine capacity bans apply to handguns. The 2013 FCCA preempted local jurisdiction over handgun regulation, but some localities claim that preemption does not apply to handgun magazines. As of 2017 this is an untested aspect of IL law.

2) NFA: Short Barreled Rifles are allowed to be possessed in conjunction with a Curios and Relics FFL. The SBR does not have to be a C&R firearm, the C&R license is just a hoop to jump through.
Click To View Spoiler
ATF has taken the position that since a trust cannot hold an FFL, that an SBR cannot be registered to a trust in Illinois regardless of whether grantors and/or trustees have C&R's. ATF also will not approve any registrations for stated OALs less than 26 inches. It is unknown whether ATF will approve a 5320.20 form to transport an SBR to or through Illinois for an SBR on a trust or if the OAL described is under 26". If you file such a form please report if you are approved or disapproved. As far as Illinois law is concerned, it appears that as long as the person actually possessing it in IL has a C&R they're ok.

Some AOWs are not regulated by Illinois law. Non-explosive DDs also appear to be unregulated by IL. More research is required on these.

Explosive devices, machine guns, SBS, and silencers/suppressors are prohibited with exceptions for certain LE uses and FFL/SOT manufacturer licensees.

3) Tasers, "stun guns" and other electrical defense devices are regulated similarly to firearms in Illinois. They cannot be lawfully "carried" in a ready to use condition as they are not included in the Concealed Carry Act. If you are transporting such a device to or through Illinois, I would strongly advise to transport it in the same way as if it were a firearm - remove batteries or other power sources to "unload" it, and have it in a case or container of some kind. If it's not possible to readily "unload" it, I would put it in a locked case and/or in the trunk so as to be not immediately accessible.

Anecdotally, there seem to be more well-intentioned people who unwittingly face legal problems from transporting stun gun/Taser type devices than who do for transporting firearms. Most people don't think about making sure their stun gun is unloaded and in a carrying case.

4) Tear gas/mace/OC type devices are limited to those 18 and over, and are restricted from college campuses and some other locations.
Click To View Spoiler
5) Certain kinds of ammunition / projectiles are prohibited in Illinois. Those include "armor piercing (handgun) ammunition", "dragon's breath shotgun shell", "Bolo shell", and "Flechette shell". See: https://ispfsb.com/Public/Firearms/FirearmProjectilesProhibited.pdf

6) Certain kinds of impact and edged weapons are prohibited from possession or transportation. Those would include "bludgeon", sap-like devices, brass knuckles, "throwing stars", and "ballistic knifes".
Click To View Spoiler
Switchblade/automatic knives were legalized in 2017 for residents of Illinois with FOID cards. The exception in the statute does not mention non-residents, but the court precedent mentioned earlier might apply for non-residents. Numerous municipalities are also reported to have local restrictions on automatic knives.

I am not a lawyer, and the preceeding is not legal advice. Hopefully this post will both reassure travellers that Illinois is not as draconian as many seem to fear, as well as keep folks out of potential pitfalls. Please feel free to post questions, additions, corrections, and suggestions. This is the current regulatory environment as I understand it, if you are arrested for breaking an IL firearm law it is not my or AR15.com's fault.

Was not able to read everything.   Spoiler tags not working on ARL site.

Might note:

Duty to inform carrying IF asked

Switchblade law has no statewide preemption and several local municipalities still  have bans, length limits, etc on the books.

This is the list of municipalities:
The other spoiler tags are just quotes from the statutes.

I added a line about the "duty to answer".

At first glance looks good. I didn't read every word.

Homewood does not have an AW or magazine ban.  That was flipped back in 2013.  The only limits now are you can't have them in public places, whatever that means.

ETA:  This is just an FYI in case anyone is local.

I'll remove Homewood then.

I think I'm going to streamline it a bit tonight, remove the transporation law quote and stuff and just put the basic instructions. See wherever else I can trim it down, it's too long.

ETA
Reordered it to put concealed carry first and streamlined it a little.

The torch is now yours, thanks for keeping an updated thread.