I'm not going to go out on a rant here and I'm sure this topic was brought up before. How would the government get away with walking into peoples homes and just take away people's guns and other property?
This isn't like someone has something that is illegal to start with like illegal drugs or something that has been used in a crime and would give the government a good excuse to confiscate the items. But when someone has a legally purchased gun, the government has no right, in my opinion, to say that gun was legal yesterday, but today we deem it illegal and we are going to TAKE it from you.
Think about it. Someone spends his hard earned money  building a LEGAL firearm collection, possibly worth thousands of dollars and the government thinks that when they send in the boys to take this collection away from him that they won't run into any problems. People kill others just for a few dollars. Now imagine the war that would result if officials started to walk into peoples homes taking away items that are worth thousands of dollars, if not priceless, especially if those items are high-cap semi-autos and the owner knows how to use them.
I can't imagine what type of ATF agent, police officer, or military person who would take sides with some gun-hating government and risk their lives to infringe on someone's freedoms and take away expensive items that were legal to begin with.

[b]ArmaLiter[/b]

Legally, it's not confiscation if the authorities are removing illegal items (contraband).  Contraband is forfeit.

If the government outlaws your legally-owned property, to serve some pressing need of government, they're supposed to provide fair remuneration.

What is "fair" is often debatable, however.  Take the case of a government using its power of eminent domain to condemn your property and home to make way for a new highway.  There have been instances where governments paid $1 to the owners before kicking them out, leaving the displaced to sue them for the fair value of their property.

The reason for all of the 'grandfather' clauses in most gun laws is to avoid the government having to pay compensation to the existing owners for their property, while cutting off new supply and (the lawmakers hope) reducing the existing population eventually to nothing through attrition.

Forgetting for a second that LE must have a warrant and probable cause to obtain that warrant in order to break the threshhold of your house (which the Sup. Ct. has held at the highest scrutiny for 4th amendment purposes), if the fruits of the search is illegal contraband, they can get it.

Is it theft?  Hell yes if the collection was legal but for some retroactive law that, ipso facto, makes it now illegal.  More than likely, the items you had in your possession before law would be "grandfathered".  If not, then I guess it would not be theft, however, you would have a legal argument that the taking was unconstitutional and that the search and seizure was illegal.