Karen Tumulty makes a good point in her piece about Hillary's next move:
So it would appear Clinton has little choice but to try to create a new dynamic. Behind the scenes, her strategists have already begun to figure out how much heat to put on the sudden frontrunner, whose win was far more decisive than just about anyone had expected.
My hunch was that the Clintons weren't blind-sided by the loss in Iowa. But the margin clearly did come as a surprise. (Just recall how vigorously Mark Penn objected to the Des Moines Register poll that predicted last night's outcome. I don't think that was all spin...) It's one thing to lose by two or three points, which may have only somewhat altered the status quo in New Hampshire. But eight (and third place) re-shapes the race in unanticipated ways.
As for the idea that Clinton needs to go negative on Obama--which Tumulty discusses at length--I just heard David Gergen make a good point (David Gergen!) about that: It often takes several days or even weeks for a line of attack to sink in, which Hillary obviously doesn't have before Tuesday. I guess that further proves they didn't expect to lose so big. If they had, they'd have already gone negative, figuring they didn't have much to lose in Iowa and would be laying the groundwork for New Hampshire.
P.S. This would normally be the place where I'd link to Frank's brilliant defense of conventional wisdom from 2001, with its fantastic riff about David Gergen. But, alas, the Internet gods have not yet blessed us with archives, so you'll just have to go to a library if you want to read it.