Well since everybody's just guessing, I might as well throw in what I think as well.
The PowerBook is the product that needs updating the most. I just bought one in January knowing this, but I couldn't wait any longer, I needed one NOW. I had been waiting for a PowerBook with a new processor since mid-last year when we expected to see a G5 pro PowerBook before a G5 consumer iMac, but as we all know, everybody who's been waiting for a PBG5 has been left with nothing for the time beeing.
The iBook is obviously beeing held back because of the lack of PowerBook updates or because we will see a full redesign. If we were to see updates next week, the high-end iBook wouldn't be more than a 1.5Ghz G4 maybe with a SuperDrive. They would just be minor updates with 512RAM standard (like the PowerBook's, PowerMac's, and iMac's last updates), unless we were surprised by a new PowerBook first. A new PB is bound to happen, but I would think until September in a future Apple event.
I think a more feasible product to be announced would be an updated XServe.
For once, all is possible in WWDC, thanks to the new measures that Apple has probably taken since previous leaks.
As a second thought I think the keynote will focus more on Tiger sales, iPods, and more sales (Jobs wouldn't miss it for the world), apps for Tiger, and other "smaller" things. The asteroid breakout box sounds like a good product, but not as a "one more thing" product, unless it's truly innovative and not just a white Apple-branded breakout box.