He's not done getting delegates, but he's done being the potential nominee. The only questions left are- how many delegates can he get and what can he do with them? To his advantage, he could do well in Southern states and they have a lot of bonus delegates. It could be a working chunk if he can last until Texas.
The big question for me is- can the anti-Romney break through in the rust belt. If Romney can't win the midwest or the South, and NY and Cal are way late in teh process, how does Mitt get anywhere close to the delegates he needs? There just aren't enough in New England and the West until California. Mitt really needs Ill., Wisc, Mich, Ind, Ohio, Penn. Does Gingrich staying in hurt Santorum's chances there (or vice versa)?
Romney has to win Illinois, Michigan, and Ohio in the rust belt, and do really well in Pennsylvania and the Texas primary whenever the hell that is.
