I think Dallas is definitely the team to look for for a rookie RB sleeper. Despite Denver's myriad of RBs, I suspect they'll work out a full time starter in the first few weeks. Given their track record with rookie RBs, if they draft a 1st rounder, say Stephen Jackson, he's worth taking as a sleeper.
Pats might have a good rookie RB, but their off. relies on Brady and spreading the ball around, not really the running game, so I'm not too enthusiastic about a RB going there.