Some Obama supporters are hoping for a Palin nomination because her high negatives will give Obama a better shot at reelection. Ned Resnikoff lists other consequences:
Palin has already demonstrated a disturbing willingness to frame even minor political squabbles in terms of "tyranny" versus "liberty," and to make her a major party’s presidential candidate would only do more to throw the spotlight on that sort of incitement. Perhaps, as Kevin Drum prays, the GOP would then, "go down to such an epic defeat that they finally get some sense knocked into them." But in the meantime, we would be facing a long, protracted campaign in which both a major political party and the mainstream press would treat violently anti-democratic positions as existing within the confines of reasonable political discourse. We’ve already had quite a bit of that over the past few years; accommodating and encouraging it could potentially make things much, much worse.
Truly: she's far too dangerous to be considered opportunistically.