PUMA's (party unity my ass) will not likely be attracted to the Republican ticket by the choice of Gov. Palin. If the reason they supported Sen. Clinton is that she is ideologically at home in the Democratic Party then they will not find much to like in Gov. Palin. Those who supported Clinton because she is a woman, and pay little attention to her politics may consider switching to a McCain vote.
Palin is more important in shoring up the conservative wing of the Republican Party. She is the poster girl for the religious, uber-conservative part of the party. This group has never been fond of McCain and there was a danger that they would just stay home on election day. That is not a problem now. Further, she may help in attracting some women who are independent, and even a small fraction of PUMA's in the right states may be enough to change the electoral map.
She may turn out to be a deeply flawed choice, but at first blush she seems a good choice by Sen. McCain.