If the code in question is proven to be 'stolen' GPL code then the seller of the code would probably be stopped from suing you over it via the prospect of 'unclean hands' -- in other words, they can't properly license code that they don't have the rights to.

Getting back to my original question: If you're simply doing comparisons between a GPL author's code and the impugned code, it might not even classify properly as disassembly under the terms of a contract, anyways. Also: depending on the venue, it's often not even legal to proscribe reverse-engineering, so it might just be a case of choosing the proper venue for prosecution of the case to avoid even having to answer that issue.

By disassembling the code you create an intermediary copy of the code. Courts have found this to be illegal (MAI Systems v. Peak Computers)!

On the other hand, courts have excused intermediate copies for certain reverse engineering purposes on the grounds of fair use, for example to to reverse engineer an interface (Nintendo v. Atari and Sega v. Accolade).

So I guess it's a question of wether what you do constitues fair use. (To reverse engineer an interface has been basically established by the courts as fair use. The DMCA gives other exceptions but I guess your case is not covered.)

But maybe there is another area of law that "overrides" IP law (Online Policy Group v. Diebold).

Boy, you need good legal advice...not mine...;-)