I think Michelle is spot on in her pre-analysis of tonight's Biden-Palin debate, but I'd add one additional dynamic that's likely to operate in Palin's favor: She is much closer to her floor of support right now than her ceiling.
At this point, I suspect, most of the likely voters who are open to being persuaded that Sarah Palin is unqualified to be vice president of the United States have already been persuaded. Though the McCain campaign did a great job of stifling the Katie Couric interview when it first began to air a week ago (though what they stifled it with was probably worse for them), it's continued to percolate for a week now, with new embarrassments--she reads "all" newspapers and magazines, she can only name one Supreme Court case--trickling out daily. Even voters who still haven't seen the Couric bits have probably read or heard about them.
In this context, I think it's fairly safe to assume that most of the voters who still believe Palin is qualified to be vice president are going to be very, very difficult to persuade otherwise. They're partisan Republicans, or social conservatives enamored of her pro-life bona fides, or people who identify personally with her and with the Joe-Six-Pack-vs-the-Pompous-Elites frame she's currently pushing. Barring some extraordinary meltdown, they're simply not going to change their minds about her.
So, Palin's polls are unlikely to go further down. But will she bounce back up? Again, my guess is yes. Where Palin seems to have dropped most precipitously is among independents, many of whom, I suspect, like her and like what she seems to represent even more. They have been grudgingly persuaded of her lack of qualifications by the Couric trainwreck, the campaign's efforts to keep her under wraps, etc. But they'll be all too happy to have her confirm their initial enthusiasm with a better-than-expected debate performance tonight. She's not going to convice them that she's the second coming of Metternich, but she may be able to clear, even if only barely, the bar of v.p. plausibility.
Could I be wrong? I certainly hope so. But for those who think the Palin pick is an embarrassment to the McCain campaign and to the country, it's likely to be a long night.