After everything I've seen, I'm pretty certain that dual-core G4 won't be around until next year, and G5s will be end of 2005 is a good estimate. I don't think they will have a PowerBook G5 WWDC, a speed bump is more likely.
I doubt the speed bump will be anything worth selling your new PowerBook over. With all the hipe about it being an "HD year" and such, a very good possibility at WWDC would be a new PowerBook 17" with a possible speed bump to 1.8-2.0 GHz, the dual-layer DVD, and possibly a graphics card upgrade (128MB 9800 Radeon?) in order to support the higher resolutions that some people have rumored about for the 17". Maybe even Final Cut Pro preinstalled. I don't think there will be an update to whole line because the 12" would not be good for doing HD, and even though the 15" could support such a speed bump, the WWDC is targeted more that the high-end pro developers who are more likely to afford and buy a 17".
Plus, most people, developers or not, don't usually buy the 17" as much as the other two (because the 17" is kind of big and expensive), so anything Apple can do to make the 17" a more attractive buy would help.
As far as PowerBook G5 is concerned, I'm willing to bet it will come out next year mostly likely, with the 970MP in it. With the heating issue, they might have the first PowerBook G5 be a 17" only, and then follow up with the 15" and 12" G5s, and all this will probably be at MWSF 2005.
As far as a Tuesday update is concerned, it will likely be an iBook update, though only a speed bump. For a form factor change, I'd say MWSF January 2005; iBook is one of their biggest sellers, other than the iPods, and for a major change, they usually would want to make an annoucement with a lot of hoopla behind it. If I had to say the earliest possible for a major iBook update, I'd say November, in order to catch the holiday crowd.
As for a Mac mini update, there is a good chance it will be at MW Paris in July, possibly with a graphics update in order to fully support Tiger.