Well, some are speculating that she wants to step down to devote more time gearing up to run for the White House in 2012, but other political insiders are saying that a move like this would probably seriously undermine her chances as a Presidential candidate.
I'm inclined to believe the latter.   She wasn't exactly highly qualified as a VP candidate last fall (considering that the VP might have to take over as President at any time). She had no national political experience and only 2 years as Governor, of a very small state (population-wise), and, unlike many other governors and US Senators, she did not have a solid background in either law or business.    Had she stayed on as Alaska's governor till the end of her term and then been re-elected, or had she gone from the governorship to maybe being a Senator from Alaska, she would have had much better credentials as a Presidential candidate 3 1/2 years from now.   This doesn't seem to be a wise move if she aspires to national office.
And resigning from your elected post without any obvious good reason doesn't inspire confidence in one's commitment and sense of responsibility.

But who knows? Maybe she'll still run for the Senate later.

Apparently she and Todd are deeply in debt due to legal expenses. Maybe she's gonna write books and be the next Ann Coulter or go on the rubber-chicken circuit as a public speaker or become a rich lobbyist for the oil or turkey industries?