With the introduction of the 2" thick iMac G5, many Mac enthusiasts' attentions have now focused on the PowerBook, and the hope of it moving to a G5 processor as well. In an interview with Tom Boger, Apple's worldwide product development director, The Mac Observer writes that Apple is unable to overcome certain engineering problems faced with making a smaller G5 chip to work in a portable.
The new iMac G5 (desktop) is thin, but (the G5) is not thin enough for a laptop right now. There are great challenges in putting a G5 processor in a laptop. The issues range from power to cooling and its overall size...You're not going to see a G5 in a laptop anytime soon.
Rumors about a G5 PowerBook have been circulating through the Mac web ever since the introduction of the original Power Mac G5. Apple has kept their position saying that the PowerBook would be a long time off, citing power and size constraints as a factor in moving the G5 to the PowerBook.