Notice "under typical workloads" in IBM's offical statements.
According to Processor Report (the free version of Microprocessor report), the thermal envelope is 20W.
Cell's POWER(PPE) core is nothing like PowerPC970 in terms of OOO processing.
They've traded clock speed for complexity, Cell should clock up to 4GHz without problems (it's held lower in the PS3 because they don't want it becoming too noisy). At 4GHz the PPE alone should kick a 2.7GHz 970's ass.
The Cell is not relevant to this discussion, it's not a desktop CPU.
What is?
Cell isn't being marketed as a desktop processor - yet.
It is general purpose though, even the SPEs are general purpose. The difference is they are optimised for certain types of problems, x86 are optimised for different types of problems.
The 970 wont be developed much further, it'll probably get a die shrink next year but not much beyond that.
That only* leaves x86 on the desktop, you really think it'll stay that way for long?
OK technically, Genesi and some RiscOS machines are on the desktop but they're not exactly big compared to the x86 market...
