Sadly, a large part of the reason XBMC can play almost anything under the sun is that it uses mplayer internally for everything but the DVD player. Mplayer is capable of loading and using many Windows codecs. Since the 360 runs on the PowerPC platform (like Macs before the switch to Intel, only dual core and faster), it won't be able to use the Windows codecs.
Much of XBMC could be easily ported to the 360, but it will be much more limited in what it can play. Also, MS quickly patched the bug in the hypervisor that allowed people to run 3rd party code, and there's currently no known way (nor is there likely to be due to some of the 360's security features) to downgrade.
If the Xbox could reliably play HD content, I'd never have a want for anything else.
BTW, PS3 is the one that uses Cell. It's open enough that it's likely that someone will get a decent media player on the thing using Linux, but it probably won't be for a good long while, as the PPE (the general purpose CPU) isn't really all that fast; it's slower than just one of the 360's two cores. That means that somebody is going to have to invest the time to refactor the decoding tasks to run on the simplistic SPEs. That's hard because they don't support all the instructions a general purpose CPU does.
The other roadblock on the PS3 side is that XBMC does a surprising amount of work on the GPU itself, and the PS3's GPU is locked down in Linux (all that's available is the frame buffer), so all the work has to be done on the Cell, which is probably possible, but much harder.