Here is my argument for what it is worth.
If she stayed on through 2010, she can not help retake the congress. She will continue to be bled dry of money through more nonsense ethics violations that did not happen.
Her popularity is greater than ever before and she can maintain a more positive media front now.
Her influence in Alaska grew from this .. see http://tiny.pl/33d8
Now she has time to write a book, make some money, start her grass roots campaign and travel making in roads she could never have done from the governors seat. Not to mention that had she campaigned from Alaska, the msm would have badgered her on that issue alone the entire time.
If she is aiming to help take back Congress in 2010, then it is a possibiilty that she is well positioned for 2012 if she succeeds in that aspect, listening to the tone of her speech it does seem that she is tired of politics in general and wants to take a different quieter role rather than active campaigning.