As there have been countless threads as to the release date of the Powerbook G5, this is well-covered territory on these forums.
To sum up those forums, here's what I remember from them:
A PB G5 introduction at MWSF '05 is highly unlikely. We'll most likely see speed-bumped G4's with a higher end video card (ATI Mobility 9800).
People have speculated a Powerbook G5 introduction at WWDC in June with simply an unveiling with availability happening later in the summer (like what happened with the intro. of the PowerMac G5).
The reason why it's been so hard getting a G5 chip into Apple's portable line is simply because the chips run far too hot to be practical in Apple's current notebook form factor. Until Apple can address these issues, we won't be seeing a G5 chip in a portable computer anytime soon.
Essentially, when a PB G5 does come out, it mostly likely will be an engineering and technological marevel, courtesy of Steve and Co. But for the time being, it's not going to happen. I'm saying that based on all the things that I have read on these forums.