I see Gephardt considering dropping out if he loses Iowa. I see Kerry dropping out no sooner than the February 3rd primaries. Lieberman may drop out then to, but I see him going on a bit.
If Edwards and Clark divide the South, which is possible, they will continue on both trying to be the anti-Dean.
Mosley-Braun will linger in a non-viable candidacy as will Marvin the Martian and Al Sharpton.
So, the most mature of the candidates will make the hard decision first and drop out.