As the owner of a 12" iBook who is planning to get an Intel Mac (although, not necessarily right away), here are my suggestions.
Don’t buy a 12" Powerbook. Buy a 12" iBook. The difference between them isn’t remotely worth the $500 price hike. iBooks are more rugged and tend to have better wireless connectivity (no metal case in the way) too.
And I’d wait until the Intel ones come out to make your decision. If they really are all that and a bag of chips, then it might be worth it to buy one, even a rev1 one. Regardless, prices on PPC laptops will fall.
Regarding the warranty, I’d wait as long as you can. The nice thing about Apple’s warranty is that you don’t have to buy it when you buy the computer. You can add the additional 2 years any time before the initial year runs out. There are a number of benefits to waiting, not the least of which may be that you manage to destroy the laptop in a way uncovered by the warranty during the first year.
FWIW, I did buy the warranty on my iBook. By the time the year was almost up, it had been shipped back twice, and I didn’t have a whole lot of confidence that it wouldn’t break again. It did end up breaking again, and Apple ended up replacing it entirely for me. So, in my case, it was worth it. I had excellent and prompt responses from Apple support, too.