"Women voters" implies a demographic that doesn't really exist. The 18 year-old female Spelman student and the 60 year-old coal miner's wife aren't even in the same ballpark on this issue. Even within this community, you've got a huge range of opinion:

Rundeep & topazz are already leaning towards/voting for Obama, so his VP choice probably makes no difference to them. Biteo and you (or an American citizen version of you) see that he's a better option than John McCain and will probably vote for him for that reason (Supreme Court), though tapping a woman might help him seal the deal. Lunesta & LaurieAnn will never vote for him, so his VP choice makes no difference to them. I think very few will be swayed as women by his VP choice.

As far as what voters in general (throwing men into the mix) will do, I think a candidate who is strong on national security experience would give Obama the biggest boost. That's where he's the weakest and where McCain will be aiming his attacks. Bush benefited from Cheney's experience in this regard.

Question for you:

Would Obama choosing a woman other than Hillary (Napolitano or Sebelius) be considered a slap? Might that not hurt his cause with some women while helping it with others? Will that be a net gain or loss?