This subject has been beat to death on the legal and full auto forums, but here's the basics...
Possession of a DIAS with an AR15 is a felony; doesn't have to be installed, possession is all thats necessary.
In US v. Cash, the 7th US Circuit Court of Appeals ruled that BATF did not have the power to "grandfather" certain DIAS's based upon when they were manufactured. The court said that the 1981 date of the BATF ruling was only relevant to a free, tax paid NFA registration of unregistered DIAS's. This free registration ended in May 1986, when all civie production of MGs was disallowed. Thus all DIAS's are now either registered NFA weapons (machineguns) or unregistered machineguns. In the latter case, if unregistered, they are a felony to possess just by themselves.
This issue has not been tested in any other US circuit though. However, that does not mean its legal. The ruling in the 7th Circuit was a well reasoned and logical decision by a well known and respected judge (Easterbrook). If tested in any other circuit, there is little doubt they will decide the same way.
There is also absolutely no question in any circuit that possession of just a post-81 DIAS is a felony. That said, the little letter that comes with the "pre-81" DIAS won't save your ass if you're hauled into court. BATF can (and probably will) do metalurgical studies on the DIAS in an attempt to prove the date of manufacture and nail you. In other words, if you get caught with a DIAS and BATF wants you, they'll get you. If you want a machinegun, buy a legally registered one. It may seem like a lot of money, but compared to spending $300/hour to an attorney to try to keep you out of prison, its cheap.