I'd give them a call to "follow up on the interview". What have you got to lose? Yeah, it will most likely be in vain and a humbling experience ("well...we didn't call you back...because we aren't going to hire you...".), but you never know! What if it was "Oh, Bill was supposed to give you a ring. Nobody called? Sorry about that. Yes, we want you to start right now..." Hey, stranger things have happened! Good luck.