Mickey Kaus raises what on its face is a nightmare scenario for Hillary: That either Obama or Edwards collapses and his supporters flock to the other candidate creating a unified anti-Hillary block which could total 40 percent or more. But this theory assumes that most Obama and Edwards supporters are in some way actively anti-Hillary, and that just doesn't seem to be the case. Today's new Hawkeye poll doesn't appear to have measured second-choice preferences, but this August Washington Post-ABC Iowa poll, back when the first-choice numbers weren't all that different, found that Obama, Edwards, and Clinton each to be the second choice of about a quarter of polled voters.
In addition, a Clinton aide tells me that large numbers of Obama and Edwards supporters specifically name Hillary as their second choice--enough to make the unified anti-Hillary scenario unlikely (although I haven't seen personally any fresh poll numbers to substantiate that obviously self-interested claim).
If people know of more good data on this question please let me know and I'll update or follow up.