I don't expect any updates to the Powerbook line for quite a while. Rumors state that the current Rev. C systems will be the last wave of G4 Powerbooks. So when they do go G5, some time from now, they will be Rev. A Powerbook G5s - Apple is notorious for producing buggy Rev. A hardware.
The Powerbook G4 is at the height of its lifecycle - all the kinks have been worked out, and it has matured into a great system.
I believe this so much that I ordered my first Mac - a 12" stock Powerbook - last Monday. I am waiting for it right now, and waiting is HARD.
I have a Windows PC as well - I don't need the laptop really until September, when I go back to University. But for the reasons stated above, and the fact that Apple's pricing rarely changes, I figured why wait?
The ONLY thing I can see happening at WWDC is a new sales promotion (the current RAM sale ends June 26th). But since I'm a student, I qualify for Educational Pricing. This usually supercedes any promotions anyways, so I wouldn't be able to take advantage of any new promotions offered. Needless to say, I'm a little anxious waiting for my Mac!