yah...your probably not going to see the G5's for a lil while...the closet that they have come to the portable size is the Xserve G5 servers, which i will say are pretty darn close to the PB size. Cooling...its defintly going to be an amazing cooling system forsure, considering the fact that the normal G5 towers have 5 fans in them and a state of the art air flow.
all i can say is if they do come with a G5 its going to be amazing...but dont expect if for about a year..maybe more ( they just released the 1.5 PB's )
Apparently according to MacUser magazine, IBM have got production problems trying to produce G5 chips with cooler running, using a different fabrication process to how the current G5 chips are made. MacUser reckons the G5 3.0 wont roll out in June, but maybe a 2.4GHz chip. When IBM eventually get the G5 3.0 rolling out, with cooler running, maybe then we'll see a G5 PowerBook with cooler running and no heat issues that gets discussed a lot in these forums.