Sorrry, I know this isn't what you want to hear, but I really think this is too hard to predict.
There are all sorts of juicy rumors about for the next rev.s of Apple laptops, but they tend to be short on specifics, especially as it regards release dates. Almost all of these have been of Page-2-calibre. 15" iBooks, widescreen 12"/13" PowerBooks, at this point none of us really know, but maybe someone has good guesses.
There have also been scattered whispers of carbon fiber as the next PB material. Doubters have added the asterisks of wireless reception and global c.f. supply problems to this particular swirl. I claim no special understanding of either issue (quite the opposite).
Talking from no particular basis of knowledge, personally I think an iBook update by the fall, or even by the summer, is highly likely. Though I have less confidence for a PB update, I have lingering doubts that they would be far behind any iBook update, if not concurrent. But really I have no idea, and neither does almost everyone else in this forum, AND neither does the beloved Buyer's Guide of "Don't Buy-- Updates Soon!"-fame.
SO I say ignore the torpedoes of doubt and just buy when you decide you need it most, which is totally up to you.