Apple got a lot of buzz for their Power Mac G5 in 2003, as did IBM for the PowerPC 970 chip inside it, made using a 130nm process in a shiny new highly automated fabrication facility. There was also a lot of buzz about the Xserve G5in early 2004, which took a while to actually start shipping due to difficulty getting enough functional PowerPC 970FX chips, made by IBM using a new 90nm process. Now there's buzz about the iMac G5, which had originally been planned for the "back to school" summer buying season, but is now expected to launch at the end of August and ship in mid-September. And of course there will be buzz about the inevitable PowerBook G5, at least until Apple announces and ships one.
Anyway, a lot of the buzz is along the lines of "G5 chips dissipate so much heat that it's hard to fit them into small spaces like 1U rackmount servers, iMacs and PowerBooks." Compared to G4 chips, this was certainly true of the original 130nm PowerPC 970. Heat dissipation is linked to power consumption, and 51 watts of "typical" power consumption per chip isn't too bad for a desktop processor, but it's not something you put in a laptop if you want anything in the way of battery life.
However, there's less certainty about the power consumption of the newer 90nm PowerPC 970FX. In Apple's recent earnings conference call, the company's chief financial officer, Peter Oppenheimer, said multiple times that the most critical issue affecting the introduction of G5 chips in, well, everything, is the need for IBM to get the kinks out of its 90nm process and actually start supplying enough working 970FX chips to Apple to meet its needs. He said this in response to questions about heat problems.
I'd just like to take this opportunity to say that the buzz about alleged heat problems is, well, a lot of hot air. :)
Currently, Apple's PowerBook G4 laptops use Motorola 7447A "G4" processors, running at speeds of 1.33 GHz (12" and 15" models) and 1.5 GHz (15" and 17" models). According to page 19 of a preliminary technical document (PDF) from Atmel, the 7447A running at 1.33 GHz has "typical" power consumption of 18 watts, and "maximum" consumption of 25 watts. At 1.42 GHz, those numbers rise to 21 and 30, respectively.
Meanwhile, the XServe G5 uses (and the iMac G5 will presumably use) IBM PowerPC 970FX "G5" processors, running at speeds of 2.0 GHz in the XServe. According to the PowerPC 970 & 970FX page of IBM's own PowerPC Quick Reference Guide, the 970FX running at 1.4 GHz has "typical" power consumption of 12.3 watts. At 2.0 GHz, its "typical" consumption is 24.5 watts.
So... a 1.4GHz G5 uses, say, 40% less power than a 1.33GHz G4. And a G4 pushed to 1.5 GHz - standard in the 17" PowerBook and optional in the 15" one - most likely uses just about as much power as a 970FX G5 at 2.0 GHz, and almost certainly more than a 970FX G5 at anything below 2.0 GHz.
So, once more - it's not about heat. If the internal design of the new iMac leads to heat problems with a G5, those heat problems would be worse with a G4 at the same clock speed! It's about Apple just not being able to get enough 970FX chips out of IBM's new fab due to the quality-control issues that tend to affect any new smaller process in a relatively new facility. Once the floodgates are opened and working 970FX chips are available to Apple in larger quantities, there's very little standing in the way of getting them into PowerBooks... and, for that matter, anything else Apple wants to put them into.
Do I think we'll see the PowerBook G5 in 2004? No. Mid-2005 is my guess, though I'd be most pleasantly surprised if they were announced at MacWorld San Francisco in January of 2005. And with the iMac G5 out of the way this August/September, Apple will have to have something big to announce then, won't they?