I think that #3 (scandal) is probably the most likely.  If you watch her press conference closely, her mannerisms are all very nervous and even more "Palin-esque" than normal.

 While it is true that the Palins are significantly in debt (a couple 100k), it just doesn't make logical sense why she can't continue to do all these speaking engagements and cavorting about that she's managed to do for the past 9 months.  Let's even take her word for it, that there is nothing for her to do until the next session of the AK legislature, isn't that an obvious excuse to continue flying around the country?

 And if her family was so strapped for cash that she would do something like jeopardize/ruin her political future, why would she artificially craft a principle that public servants should resign when they are ineffective (an idea few agree with or at least would never hold her to)?  She should have just kept her AK payrolll, transportation accommodations, health insurance etc etc.  She's got two infants and a pile of debt and now no health insurance, isn't that a totally irrational position to put yourself in?

Now, in engaging in speculation on what scandal is the root cause of Palin's departure, I think we should take stock of the times and go with the wildest suggestions possible.  Here are some ideas I've heard floating around and also some of my own

1) She's pregnant again.  Obviously this makes her look like a lunatic and would really jeopardize her political future.  Maybe she thinks she can hide it like last time or at least downplay it.

2) Her youngest daughter Willow Palin is pregnant (hey don't put anything past these Palin's when it comes to birthing).

3) The federal investigations Cenk mentioned/ the AK police chief probe / any of the alleged corruption.  These all seem possible, but I think her control over them would be greater if she remained governor.

4)  Levi Johnston's "tell all book."  News of this book coincidentally broke at the same time.  Maybe Levi is going to reveal some terrible secret and the Palin's people caught word it was coming out.  This might be even more likely if the secret involved: 4 b) Sarah Palin's son Trig was actually Bristol's child (there were rumors of this during the campaign).

 If Republicans have taught us anything as of late it's to expect the worse.