what Major Nelson told kotaku.com:

A peek at Gamercard aggregator MyGamerCard.com reveals a long list of Xbox 360 users already racking up achievements on Grand Theft Auto IV since an illegal torrent of the game was leaked yesterday and early copies started showing up in people's homes.

Neither Rockstar nor Microsoft itself have yet responded to Kotaku's requests for comment.
Xbox Live's Larry "Major Nelson" Hryb, did say, however, that he was unable to comment yet due to the "need to look into this further," so we can assume that Microsoft, at least, is investigating.

Any XB360 users that downloaded and played before the release date, while connected to live will probably get banned...