Wow. THe lack of knowledge of how hardware and software work together in this thread is apalling.
For one, Windows will NEVER in 312954639847 years run on either the PoS3 or Xbox360. It is not possible. Windows would have to be completely, and I mean COMPLETELY rewritten from the ground up. The last and only windows OS to be able to run on any other architecture other than x86 was NT. I have 3 DEC Alpha's that run NT4.0 quite nicely, but that's the only version of Windows that will run.
Now, onto the PowerPC side of things. Don't expect linux or OS X to run on either of these consoles. They are both In Order Execution processors, so even if those OS's could run, it would be incredibly slow compared to even a G3 system. Not to mention the fact, that you are banking on the fact that Xenon and Cell are using the same instruction set as the G3 through G5. They are both custom CPU's. They each will have specific instructions (think SSE or AltiVec) to optimize performance.
Next, lets take into consideration that the PoS3 could run linux. Even if it could you would only be using the PPE. Why you ask, I mean Linux IS a multi threaded OS is it not? Right you are, but the SPE's are designed to be specifically coded for and will not just work like if you buy a server motherboard and throw 2 processors in or use a dual core processor.
Now, believe it or not, but the X360 could be even more difficult because it has 3 cores. Not only are the In Order, but that's a weird number for cores, and multithreaded OS's require processors to be in a certain config. 2, 4, 8, 16, etc. 3 would not work right. Plus, the shared L2 Cache would throw everything for a loop as well, not to mention as someone mentioned earlier drivers. The GPU in the X360 acts as a northbridge as well.
With the original Xbox drivers weren't such an issue. My xbox runs linux fine, and linux sees the NB as a Nforce.
Also, here is a link to a picture of the cpu and gpu on the X360 motherboard, to show you the GPU as a NB.
Click me.
EDIT: I'd also like to add the Xenon is NOT a G5. It never was.
EDIT2: Didn't notice the post above mine. No one has gotten OS X to run on the Xbox. They have gotten darwin to run. The Xbox would be next to incapable of running OS X due to memory restraints. Also, the X360 has 512 MB's of shared memory, similar to the way the Xbox has 64 MB of shared memory. The PS3 will have 256MB system memory and 256MB video memory.