You think it would require much more work than just a recompile? With the similarities between iOS/OSX and Apple's investment in the LLVM toolchain I see the switch going smoother than PPC -> x86.
That said, the entire computer industry is going ARM (Windows 8, all mobile platforms, etc.) so I'm guessing that the transition will be easier.
There's much less tight x86 assembly for critical performance areas than was commonplace 10 years ago.