Palin’s own candidacy depends on Perry’s popularity. If Perry remains popular and grows, Palin will not risk getting in and divide Tea Party votes. So, when she is forced to pick someone (against Obama), she will probably side with Perry. Romney is not going to win the nomination, so picking Perry is the obvious choice for Palin.
However, if Perry’s popularity stagnates at all, I expect Palin to jump in and sweep everybody away. I think she is viable as a potential candidate until late fall of 2011... possibly November even.
That is really a silly statement because "late" fall doesn't occur until Thanksgiving or after, which is late November. Prior to November you are talking about "early" fall.