Two great quotes from Electoral-Vote:
A new Washington Post/ABC News poll shows how polarizing Palin is. Among Republicans, 76% have a favorable view of her but among all Americans, 60% say she is not qualified to be President. The bottom line on numbers like these is that she could conceivably win the Republican nomination, only to be squashed like a bug in the general election.
Another factor to consider is the possibility of a reverse Limbaugh play. In 2008, after it was already clear that Obama was going to be the Democratic nominee, Rush Limbaugh strongly urged Republicans to switch their party registration to Democratic to vote for Hillary Clinton in order to prolong the primary process. It is not clear how many did, but little did he realize how much he was helping Obama. By forcing Obama to hone his message and campaign team in the Spring, he had a well-disciplined organization in the Fall. If Clinton had dropped out in April 2008, Obama would have been a far weaker candidate in the general election. In 2012, we might see the reverse, with Daily Kos and other Democratic-oriented groups urging Democrats to reregister as Republicans in order to vote for Palin in the primaries. However, this is where the comparison ends. Palin could actually win the nomination and would be the weakest possible candidate the Republicans could field in the general election.