Santorum's going to have to win California to win the nomination.
Just ran the CNN calculator - at this point, I honestly think Santorum would have to win almost every May and June contest, and could definitely finish strong in April (A comfortable Pennsylvania win, staying competitive with Romney elsewhere and possibly a win in Delaware). California is a state I think Santorum will need to win in order to keep Romney from getting enough delegates. I don't think there's any realistic way that Santorum claims enough delegates - he's either convention or bust at this point.
Today, FOX said he'd have to win 80% of the delegates from here out to win the nomination. If you assume Gingrich's delegates go to him, it ends up being a bit less than 80%, but it's not going to happen.