Only if you hacked your Xbox and can run a linux CD off it.
The xbox does not have any sort of NAT/routing software embedded into it.
Thanks. :-) Follow-up Question:
Is there a such thing as a *cheap* one device powered router(?) or switch(?) or *something* that I could plug my Raspberry Pi into via ethernet port such that the *thing* used whatever power, etc. was needed to get to the livingroom router that is wifi enabled ?