IBM's POWER series server chips are pretty specialised and probably cost more than a PowerMac for the chip alone.
Note that the G5 is based off IBM's POWER4 architecture, the chip you are talking about that IBM is coming out with is the POWER6 architecture. The POWER5 architecture has been available for a while, so if IBM really cared about Desktop chips we should be on the G6 by now really.
Personally I think that IBM find attempting to be competitive with Intel and AMD too much of a drain on resources and they would rather make money from high end/expensive chips and chips for consoles that have lifetimes of 5-7 years in manufacturing, but generally never get any faster.