From what I understand, she is governor of a state with fewer people than Barack Obama's state senate district in Illinois. She has served less than half her term, and her largest elected role before that was being mayor of a town of 3,000 people. And I hear she's currently being investigated for abusing the power of her office.
What would choosing her do for the GOP's claim that Obama is not experienced enough to be president? After all, they'd have her, with arguably less experience, waiting in the wings to take over if 72-year-old McCain croaks in office.