x86 emulation on PowerPC should be quite reasonable if using a JIT implementation - I say reasonable in the comparable processor sense. Don't expect AthlonXP performance on a 603e :-) Given that the PPC has more gen purpose registers than the x86, I could forsee a really nice JIT that actually keeps the x86 register set in the PPC during execution of the code - kinda register allocation architecture taken to its logical conclusion.
Perhaps on Altivec systems some clever person might find a way to emulate some of the x86 SIMD stuff [mmx/3dnow].
