The PowerPC 980 Roadmap
First hints of a new derivative chip came from this extensive roadmap of the PowerPC, Apple, Motorola and IBM from November 2002. At that time, the PowerPC 970 had been announced by IBM but not yet publicly endorsed by Apple. Apple later announced PowerPC 970 based PowerMacs in June of the following year (2003).
Today, Appleinsider reports that the PowerPC 970 will max out at 2.6GHz while the Power 980 will come due in Q3 of 2004 and bring the PowerMac up to and beyond 3.0GHz. The 980 is reportedly a 90nm chip and may be branded as the G6. Appleinsider also comments on updates to the subsystems of the upcoming PowerMacs offering significant performance gains.
This "980 @ 3GHz" timeframe was echoed by a detailed report from July 2003. In that report, Apple and IBM were said to be working on parallel development of the Power5 and PowerPC 980. That report also claimed that the 3GHz PowerMac would not arrive by way of the 970 -- which was said to top out at 2.6-2.8GHz, while the 980 was expected to start at 2.6-3GHz (from the July report).
A subsequent report noted that Apple was already given PowerPC 980 samples in October, and also made predictions about 90nm 970 based PowerMac updates in February at 2.5-2.8GHz. This latest report also offered significant details regarding cache, SPEC scores, and bus multipliers on the as-yet-unannounced PowerPC 980.
Without any tangible releases from these rumors, it's hard to gauge their accuracy, though the information appears to be consistent -- though it all may simply represent a common source.