I would not be surprised to see a Romney/Santorum ticket, and I thought so even before Iowa. If Romney gets the nomination--still an "if"--he will need to pick a social conservative to reassure the base. And Santorum did endorse and campaign with Romney in 2008.
I’d thought the same for a while as well. Although Santorum is from PA, functionally he is like a political southerner balancing the ticket of a New Englander.
Santorum’s surge and, I think, run-for-it will define the race from now on.
As I see it, the closer Santorum gets to overtaking Romney, the more okay with a Romney/Santorum ticket people are going to become. If Santorum captures a large “market share” and, therefore, can help in a big way to deliver the presidency to Romney, it *should* mean that conservatives have a good stake (place at the table) in the administration.
I also thought it was possible that Santorum could take the nomination away from Romney, but the trends are otherwise. Romney has been accepted, just not acceptable, for a while. With Newt crashing (which, I think, is irreversible, but we’ll see) and Santorum showing a strong pulse, it’s almost as if the electorate suddenly said, OKAY, we’ll go with Romney already if we can get a strong conservative such as Santorum onto the ticket as well.