x86-64 is part of the cross-license agreement, so it's unlikely Intel paid AMD anything for it. It's the same for things in the other direction, like Intel's SIMD extensions that AMD uses.I would not worry about it. However, I am more concerned about what would happen in the console space. Also, AMD created the X86-64 extensions, if AMD goes out of business, I hope Intel will have to buy those extensions or stop using them. Otherwise, AMD made a horrendous deal with Intel and they deserve to die.
The x86/x86-64 instruction set isn't really what's licensed, but the techniques and circuits to implement it can be under IP. The claim to x86-64 in Intel's processors might have been stronger for Prescott and later Netburst CPUs, but it's been completely reimplemented in Conroe and later CPUs, most likely to free Intel from any claims AMD might have had in the future. So no, there's probably nothing AMD can demand money for about x86-64, but that doesn't really matter because...
It's funny you bring it up, but AMD basically loses claims to everything if it's sold or goes out of business. See sections 3.9 and 5.2 of the current cross license agreement: http://www.sec.gov/Archives/edgar/data/2488/000119312509236705/dex102.htm lol, it seems AMD isn't exactly in a powerful bargaining position.