Guess it’s not just the Xbox - http://www.0xd6.org/ps2-independence.html
You can buy a linux kit and run whatever the hell you want on a PS2. Link’s down, though.
I have one of these and I don’t think this is strictly true (you can’t run arbitrary discs; you can only run Linux executables).
I never played with my linux kit all that much though, so I could have been missing something.