Not only will Sarah Palin not seek a second term as governor of Alaska, she's resigning the office, effective later this month (hat tip, as is so often the case, goes to memeorandum).
I've already seen some speculation to the effect that she's "freeing herself up for a long presidential campaign," but c'mon -- that just doesn't make any sense. "I bailed three quarters of the way through my only term as a governor" isn't good presidential stump speech material. She might be contemplating a Senate campaign next year, but that wouldn't be much better. Quitting a job isn't the way to get promoted.
Nor does it strike me as likely that she's breaking under recent rounds of criticism/attack. Love her or loathe her, she just isn't the type to wither under that kind of fire.
Best guess: Palin, or a close family member, just got very bad medical or legal news. I could be wrong -- or to put it a different way, she could be stupid -- but it's the only thing that makes much sense.