Here's my take on how that affected Romney:
Seems to me, based on all I've read, listened to, talked about with people, that the Santorum surge "allowed" people to feel more comfortable with Romney. Easiest way to explain it.
They're not yet sure (and may never become sure) that they want Santorum to be President at this time (he's young and just becoming more well-known nationally), but they like the fact that he is challenging Romney and, at the minimum, is now the likeliest VP candidate if his momentum continues.
With Santorum in the picture, as a sort of check on Romney and a potential balance to the ticket (as Palin was to McCain), it looks like voters are more willing to support Romney, especially as he looks to many as inevitable anyway.
So, in my view, the better Santorum does, the more voters are also going to support Romney. It's sort of like saying, "okay, I'll go with Romney now that there's a true conservative who we can make the case for as VP."
Of course, some could make the argument that the Romney surge is a reaction against the Santorum surge, but that's just not the sense I'm getting.
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.