Rocking in his comfortable, leather office chair, Microsoft's Larry "Major Nelson" Hyrb scours the landscape viewed through his widescreen computer monitor, his hand gently resting on an enormous and ominous banhammer.
... okay, maybe it's not Major Nelson personally banning accounts (although we love the scene we've envisioned), but the director of programming for Xbox Live has delivered a rather stern message to the community about the consequences of sharing and/or tampering with Xbox Live accounts. We're not sure what exactly prompted the public warning, although there is a specific reference to people sharing gamertags for the purpose of earning Halo 3 skulls (tsk, tsk people).
The short answer: just don't do it. If you need more motivation, Major Nelson outlines what will happen should Microsoft have to go to the pain on you.