Are you sure about the G5 heat dissipation, especially the newer PowerPC 970FX? My understanding is that the G5 isn't a whole lot hotter than the G4. The huge heatsink and fans in the G5 PowerMac are designed that way to allow near-silent cooling of the entire system, rather than to cope with a furnace-like heat output from the G5. The G5 certainly runs much cooler than some of the Pentium 4 chips currently used in shipping x86 notebooks.
According to an article on the PowerPC 970 from arstechnica (link below), the original 0.13um 1.8GHz G5 CPU produces 42 watts...compared to the 2.8GHz Pentium 4 which produces 68.4 watts, and the 1GHz G4e which produces 30 watts. So you're looking at less than a 50% increase in heat output for an 80% increase in core frequency. It also mentions that at 1.2GHz, the first-gen PowerPC 970 produces only 19W...11W less than a G4e running at a 200MHz slower clock speed. The newer 0.09um 970FX G5s will be cooler than this, but as was highlighted in the recent Apple financial results conference call, is being held up because of manufacturing delays at IBM.
I should just clarify this...I'm not saying that heat issues with the G5 probably aren't going to be a major factor in the redesign of the PowerBook G5...I'm just saying that from what I've read, it doesn't seem that the G5 chip actually is 'the mini furnace' that many people claim. It's actually quite a bit cooler than CPUs already being used in common x86 notebooks. It shouldn't be an insurmountable issue for Apple...and I think 'G5 heat issues' are a red herring in a lot of product discussions around here too. As I mentioned before, it seems that PowerMac G5 updates are more likely delayed because of chip shortages from IBM, rather than the oft-repeated reason in these forums of "Apple can't keep the faster G5s cool!!!! OMFG!!!! HOT HOT HOT!!!!"
Does anyone have more up-to-date heat dissipation figures for the PowerPC 970 and the new PowerPC 970FX?
http://www.arstechnica.com/cpu/02q2/ppc970/ppc970-1.html