Short answer? None.
Long answer? Read his full analysis here. An excerpt of how he breaks it down:
Actually, Mr. Romney has a bit of a problem. The Republican women with the most traditional qualifications for the vice presidency tend to be moderates, especially on abortion choice, probably making them unacceptable to the Republican base. Another group of up-and-coming female governors and senators may not be adequately seasoned for the rigors of the campaign trail. The few exceptions are probably too old, or too controversial, to be smart choices with swing voters. It has nothing to do with their gender, but any of the women that Mr. Romney might choose would be at least a little risky.
Then again, when I watch stuff like the new Political Animals, all I can think about is how primal people’s quest for power can be. Unsavory doesn’t begin to describe it.