Nate Silver at fivethirtyeight.com suggests that "there is a small but nontrivial chance that the Republican nominee could be someone like Jeb Bush, Paul Ryan, Mitch Daniels, Tim Pawlenty or Chris Christie."
The theory is that a brokered convention may be necessary to gather support behind a single candidate, allowing for a Republican who is not currently in the race to enter and possibly win the nomination.
"But the odds of a dark-horse candidate or brokered convention will increase to the extent that any of the following conditions hold:
1a. Mr. Gingrich leads the delegate count, but does not have more than about 50 percent of delegates.
1b. Mr. Gingrich holds more than 50 percent of delegates but is involved in a significant gaffe or scandal at some point later in the campaign.
2a. Mr. Romney has performed poorly enough in the early states that he is no longer viable.
2b. Mr. Romney is viable but his unfavorability ratings have considerably increased to the point that he no longer qualifies as a consensus choice.
3. A factional candidate like Ron Paul holds 10 or 15 percent of the delegates.
All of these conditions seem plausible based on what we know right now, which is what leads me to believe that a brokered convention is plausible as well."