NationalJournal has a poll of 50 Dem "insiders" of who they think will be VP. The top 10, in order:
Edwards
Gephardt
Vilsack
Richardson
Graham
Bayh
Nelson
Clark
Kerrey
McCain
The link: http://nationaljournal.com/members/campaign/2004/whitehouse/deminsiderpoll.htm
It's interesting that even as the primaries fade in memory that 4 of the 10 most likely (according to these people) were Kerry's presidential rivals. That's only happened twice in both parties in the last 50 years (Kennedy-Johnson and Reagan-Bush). On the plus side, both of those tickets went on to win and both running mates succeeded the top guy as President and won exactly one election in their own right.
Also interesting to note that the insiders figure it will be the Midwest VP strategy (a credible case has been made that Edwards will have more impact in the Midwest than the South) rather than the Southwest (Richardson) or Florida (Graham or Nelson)