Jonathan Bernstein imagines it:
In most GOP caucus states, the voting is not strictly connected to delegate selection. If Santorum’s voters don’t understand the procedures, it’s very possible he could win the vote and yet pick up only a handful of delegates. … It’s not impossible — though it’s very unlikely — that the popularity contest could leave Santorum as the clear, unambiguous winner, while Romney becomes the clear, unambiguous nominee. Imagine Santorum finishing with a five point edge or more in votes — even as Romney gets crowned the GOP candidate for president. If that happens, it’s hard to see rank-and-file Republicans accepting the outcome as legitimate, even if the Republican partisan press tries hard to sell it.
Sean Trende revisits the brokered convention scenario with an interactive spreadsheet.
The Dish 