IBM has determined that there is a niche they can fill that won't be eaten alive by the attack of the low end processors (x86, in this case). That niche is one in which they use exotic chip packaging (and cooling systems) and extremely high bandwidth system interconnects to stay ahead of the mainstream performance. POWER5 has 2313 signalling pins. Desktop CPUs have only recently approached 1000 total pins (signalling + power).
If you detach the POWER5 from it's remarkable cache+memory hierarchy, it suddenly looks a lot less interesting. If you don't detach it, you end up having to pay the rather huge costs of routing and connecting all those high speed busses. POWER6 will continue this with no fewer than 9 distinct bi-directional interconnects totaling 300GB/sec of bandwidth when running at 5GHz. While this is likely to once again bring them to the front of the markets they're targeting, there's no way a desktop motherboard vendor could hope to support it.
As for CELL... wait for the PS3 to be released, and watch the benchmarks of linux apps on it. I predict you'll see a few of them go blazingly fast, and the rest be around the level of the G4. Current programming paradigms are simply unable to handle CELL's low per-thread performance, asymmetric design, and exotic memory management requirements.
