I think it would say different things to different people. Some would see it as admitting Biden was the wrong choice initially, so it might be twisted as weakness. Also, I know a lot of people who really don’t like Hillary, but I am going to guess they likely would not vote for Obama anyway.
I admit that if Hillary was Obama’s running mate, It would probably lock in my vote for Obama. Right now I am kind of waiting for the Republicans to get their guy, and once they choose I will seriously listen to everything they have to say. If Hillary is on the ticket I will probably be less attentive to the whole process, and less easily swayed to consider the Republican nominee. If Hillary were running for President I might hyperventilate I would be so excited. But, as VP, I think she will have little influence to be honest on the decisions Obama makes. Right now he can consult with the Clinton’s any time he wants I would think.
For Hillary, it would be nice for her to not have to travel around the world for another 4 years, she has an incredibly exhausting position. I wonder if she steps down from it, and was not involved with politics for a few years, if it would hurt her chances of winning if she ran in 4 years.
My conclusion is, I think putting Hillary into the VP spot would probably not harm Obama and could help. Maybe he should have done the switch a year ago before the election process started to heat up.