RealWorldTechnologies posts an in-depth review of upcoming 64-bit processors.
In it, they discuss IBM's Power4 and, of course, IBM's PowerPC 970 which is coming later this year:
Even more intriguing for Apple is that the 970?s typical power consumption drops to 19 W at 1.2 GHz which makes it a natural competitor to Intel's Banias processor for high end mobile applications and very small form factor and/or silent desktop PCs. Given the reduced design margin and greater market emphasis for clock frequency of desktop processors, it is also conceivable that IBM could turn out limited numbers of 970 MPUs that clocked at 2 GHz or higher for high end desktop Macs, an important psychological milestone for Apple's struggle for survival in an increasingly x86 dominated PC world.
While the MHz boost is purely speculation, Apple is aware of their lagging processor speeds (as compared to Intel/AMD), and IBM's PowerPC 970 is widely expected to be incorporated into PowerMacs as early as later this year.
