I think it's still a bit of fantasy that the convention would actually be a contentious one with two or more potential nominees -- the party won't allow it, ultimately -- but it's always fun to speculate about the notion that Romney doesn't come out of the primaries with the 1,144 delegates needed for the nomination. Philip Klein:
It is conceivable to craft the following general scenario: Romney wins states in the west and northeast, Newt Gingrich and Rick Santorum combine to dominate the south, Midwest and Appalachia, and Ron Paul siphons off a chunk of delegates. If such a scenario plays out, it’s possible to see how Romney could have problems getting over the top, even if analysts are broadly correct that his money and organization give him the edge.
Ron Paul is evidently absconding off with delegates surreptitiously. He still doesn't have a chance, but he could be the monkey in the wrench here, yanking away delegates from the viable candidates.
This becomes more fun by the day.