The GOP has a possible 2286 delegates. Of those, 745 have already been awarded, with 415 going to Mitt Romney, 230 to other candidates.
You need 1144 delegates (1 more than half) to win the primary. As Romney currently has about 55.7% of the delegates, to the uninformed it looks like he will make it.
Unfortunately for him, the reality is a bit different.
First of all, the Santorum campaign got off to a rocky start and could not even try for Virginia (did not make it onto the ballot). But it's been picking up steam and they won't have that problem again. More importantly, Romney's has used up almost all his safe votes. He is governor of Massachusets, and grew up in Michigan. Obviously those would be his two strongest states. He is also Mormon, which helps explain his wins in Nevada and Arizona. In addition, he won New Hampshire, Maine, and Vermont. He really has only one more safe state - Utah is the only remaining strong Mormon state. Yes New York touches Massachusetts, but Santorum is from Pennsylvania which also touches New York.
If you pull out the safe states I mentioned above, (244 votes for Romney, plus 59 votes for other people), then Romney has a win ration of only 47.37%.
Assuming Romney wins all of Utah 40 delegates (Mormon), but none of the 71 Pennsylvania delegates (Santorum's home state), that brings him up to 455 votes. Then if he wins 47.41% of the remaining 1430 delegates, that brings him another 678 other delegates, for a grand total of 1132. Which makes him short by 12 whole delegates.
The odds are very good for a brokered convention.
Note that Romney will still most likely WIN the brokered convention. He has more than twice as many votes as Santorum, or a more than 200 vote lead. Santorum would probably need to convince Gingrich AND Ron Paul delegates to support him in order to beat Romney, while Romney needs only one these groups to win.