I'm pretty certain Governor Palin did not sign on with John McCain so that she could become a doormat. Given that it's pretty clear that she's more popular than he is, I would expect they both realize that--were she so inclined--she could severely hurt John McCain politically. Of course, neither she nor McCain would want things to come to that, but that doesn't mean there aren't any circumstances where she might need to make a credible threat.
While previous VPs have not made such threats, at least not in recent times, that's largely because previous VPs have not been anywhere near as popular as their running mates. Palin's popularity caries with it an unprecedented level of political clout for a VP.
We will see. McCain will win this election and the Democrats will hold the Senate. Within the first six months of the McCain Presidency a Stem Cell Research Bill will go to the Floor of the Senate.
When it is time to Vote the Senate Democrats will manipulate their votes so that their is a tie forcing the VP to cast the tie breaking vote in favor of Embryonic Stem Cell Research just as John McCain favors.