Microsoft will dig themselves into a hole if they call it Xbox 360. What are they going to call the Xbox 3? The Xbox 720? My favorite name is Xbox: Reloaded.
Plus the fact that it has "box" in the name kinda limits their future design choices don't ya think? I mean there aren't too many round boxes lying around.
Calling it XX or "Double X" would make it inferior to Van Disel's character Triple X.