Differences in polls seem to be related to whether the sample is of registered voters or likely voters. The same pattern appears in New Hampshire.
This is from Gallup, but I don't think it is free yet.
http://www.gallup.com/poll/talk/default.asp?pw=9/9/2003The National Polls
Four prominent news organizations conducted national polls of Democrats in August. Three of the polls show Lieberman ahead -- with leads of 4 to 10 points over his next-closest rival. The Zogby poll, by contrast, has Dean in the lead -- with the other three major candidates essentially tied for second.
Lieberman
CBS News (Aug 26-28) 14%
CNN/USA Today/Gallup (Aug 25-26) 23%
Zogby International (Aug 16-19) 10%
Fox News/Opinion Dynamics (Aug 12-13) 18%
Gephardt
CBS News 10
CNN/USA Today/Gallup 13
Zogby International 11
Fox News/Opinion Dynamics 12
Dean
CBS News 11
CNN/USA Today/Gallup 12
Zogby International 17
Fox News/Opinion Dynamics 11
Kerry
CBS News 5
CNN/USA Today/Gallup 10
Zogby International 9
Fox News/Opinion Dynamics 13
<snip>
Dean Fares Better Among People Paying Close Attention to Nomination
One of the major differences in the polls is the identification of the sampled population. CBS, Gallup, and Fox all interviewed registered voters who identified themselves as Democrats. Zogby interviewed likely Democratic primary voters nationwide. This characterization implies a subset of registered voters, though Zogby does not indicate how the likelihood of voting was measured.