One thing that might complicate Apple's transition to x86 is availability of 64-bit mobile CPUs. Intel have recently announced that Yonah, their next Pentium-M core, won't support x86-64 instructions. However, it'd be awkward for Apple to target 32-bit x86, and have to go through another transition to x86-64, even though that one is mostly transparent. Yonah is due out Q1 next year, and Intel are currently saying that they'll offer 64-bit mobile CPUs "when the market requires it".
I'm not sure what Apple currently do for 64/32 bit... they only seem to offer one set of install media. Do they just detect a G5 and install a 64 bit kernel and libraries if required? What about the rest of the OS, are the binaries there 64-bit or 32?
(The Pentium-M is quite an impressive bit of kit, and should help any worries that Mac users traditional complaints that PCs are hot and noisy. The 755 model gives over 1500 SPECint with a maximum power possible output of 33.5W).