It's a rumor going around and Romney has hinted at the desire. I am going to just come out and say that a McCain-Romney ticket is powerful and makes a LOT of sense. It's imho a no brainer.

Huckabee would be a whole lot more likely, I think. VP candidates are usually chosen with the intention of shoring up a candidate's weakness, and McCain's weakness is with the evangelical vote. For him to have a prayer of even seriously competing in the general election, he's going to have to pick someone with some serious religious-right cred. Romney not only doesn't have that, he'll actively alienate them more that McCain already does.
I disagree that it's a requirement to attract the evangelical vote. McCain is probably one of the least religuous republicans running for President in a long time.