You might wanna read up on the macnn.com, apple support forums too. There's different "quirks" associated with the 15" and the 12". On the 15" - latch problem, white spots (though I think they resolved this eventually), and some other smaller issues. On the 12" - Wobbling, case warping and a bit of a heat issue. Personally, I had wobbling the first day but it went away, but the case warp is there, but very very little. Hoping it won't grow.
But I'm loving my 12", it's a great backup to my PC.
Oh yea, the powerbooks are due for a speed bump/update soon, like before the end of April soon.