I think his behavior is all the evidence one needs. Santorum is exactly like Romney in everything but his more conservative social positions (many of which became a lot more conservative when he saw that they were selling points-look at his track record).
To me the depressing thing about this is that it shows that social conservatives are by no means political or fiscal conservatives, and that GOP voters actually do want a big-government nanny-state RINO.
On that basis, I think Santorum believed he had the VP position sewed up, since Romney would benefit from his social conservative votes and Santorum thought this would be a bargaining chip. But I think Romney now realizes that he doesn’t need them, and that most of the socons will vote for him anyway once he becomes the nominee, so now he’s free to look for another RINO from an important state (such as Rob Portman from Ohio).