Don't expect G5 powerbooks any time soon. Apple still has far too many G4 CPUs and G4 motherboards in the warehouses that they have to dump on the masses. There really is no valid technical reason why the G5 couldn't be in a powerbook now, other than the Osborne factor.
Remember the Osborne computer? Great little machine which was released in 1981. It was an instant success, selling over 10,000 units a month - in 1981. By September 1981, it was doing $1Million in sales a month. The company was successfull beyond anyone's wildest hopes.
In 1982, while the company still had a warehouse full of Osborne-I computers, and many more warehouses full of parts, they announced a planned successor for the next year - The Osborne-II with greatly improved capabilities. Sales of the Osborne-I virtually ceased, and by 1983 the company was bankrupt.
So, as long as Apple has many many G4 parts to get rid of, don't expect a G5 powerbook. AND as long as the masses keep chanting the mantra - "G5 in a powerbook can't be done, too hot, too large, too much power demand" Apple will happily peddle yesterday's technology at tomorrow's prices.