It depends on Intel
If ZDNet's latest article is anything to go back (the hacked OS X on a Pentium feature), the OS is ready, the only real issue is going to be the lack of Altivec in Rosetta, and Intel seems to be roaring ahead of schedule on transitioning to 65 nanometer chip fab. Yonah, on the Napa motherboard, will be launched in standard performance single core and high performance dual core in early January -- only the ultra low voltage version (presumably for subnotebooks) will have to wait.
Under the circumstances, why would Apple wait? Time to move all the mobile lines to Intel, and by proxy that also includes the Mac Mini. A subnotebook, the only thing held up on Yonah, is the only thing Apple currently lacks in their lineup anyway. Dual core Powerbooks (which will have just enough oomph to deal with the lack of Altivec in Rosetta for PPC-compiled Adobe stuff without a major slowdown, but will make the iApps, Office, and non-Altivec programs like Stata and SPSS all do well), maybe even a dual core mini, and single-core iBooks. The more progressive developers like Stata will have optimizations out very quickly. The presence of a dual-precision SIMD on Yonah is further reason to move quickly; within months, graphics and even more notably statistical apps (which can't use single-precision Altivec) will get a huge new lease on life.
G5 is a different story. No immediate rush there, but I'll bet the iMac goes over by fall.
I say Apple is ready on software just on the basis of ZDNet's slightly ethically-challenged but interesting story about running hacked OS X on a Pentium and finding it as fast if not faster in the OS, while, given the iTunes test, still having obvious trouble in Altivec. But on a dual-core, no one should notice too much.