I really can't see what you would lose in buying a new mac this fall? What is the lifespan of an iMac? 3-4years, a PM 4-6 if you are not depending on them for a living. The transition will take 2 years, and im guessing all Apple software will be compiled for the ppc platform for atleast 5 more years after that (think about those who will buy the last week before the end of the transition, yes i bet there will be those!). This means if you buy a Mac this fall you are sett for atleast the lifespan of the hardware. Then at next update you get a x86 Mac.
Buying a new iMac PM hasnt looked better than now, since it will easily carry you effortless through the transition.
Just my opinion, and im getting a PM this fall!