I really don't think that the 17" is all that heavy, I have one and I carry it around all the time, I'm not what one would call "muscular" either.
I don't expect a G5 laptop in a long time. My 17" 1.67 Ghz G4 runs quickly, and silently. And, it doesn't get that warm. It does the job, and even if a G5 were to come out, I don't know if I'd fork over more money for it, I'd upgrade to 2 GB ram first, then, maybe in a while it might be time to upgrade, but by then, there will be another chip out! The current PBs are no slouches for speed.
Just curious though, how does the fan system for a laptop work? When I look at my PB, I see wow, alright, the top half is the screen, the center of the bottom is battery, the right side is CD Drive, they fit a hard drive in there somewhere, a processor... These things are pretty compact already.