Taken from the x86-64 article on wikipedia:
This has led to a case of mutually assured destruction should either company revoke its respective license. Should such a scenario take place, AMD would no longer be authorized to produce any x86 processors, and Intel would no longer be authorized to produce x86-64 processors, forcing them back to the now-obsolete 32-bit x86 architecture.
http://en.wikipedia.org/wiki/X86-64
I would think AMD are safe for now.