Eventually Dean will be in a 2 or 3 man race, though, and thus as you say he will have a much harder time winning. Right now he actually only has about 20% of Dem voters in polls. So in order to win he will have to pick up significant support from the backers of other candidates once those candidates drop out. If all of the Kerry, Lieberman, Clark, Edwards, and Gephardt people don't like Dean, then there is no way Dean can get nominated.
I'm of the opinion that if Dean wins in Iowa the race is his. Because of the extremely front loaded primary calendar, once Dean starts winning I don't think he's going to stop--even after the field gets whittled down to two or three. If Dean does what I think he might, which is knock out Gephardt in Iowa, Kerry in New Hampshire, and Edwards in South Carolina, there's just no one left to stop him. Clark is a lightweight and I don't even think he'll be a significant factor by the time South Carolina rolls around and Joe Lieberman is just too weak of a candidate to put up much of a fight. And even if Lieberman manages to win Arizona, Oklahoma, and some of the other Southern/Southwestern primaries and caucuses it's not going to amount to a first ballot victory at the convention.
I think Lieberman (and the dems) only hope is that they can rally behind Lieberman after Dean has won New Hampshire and Iowa and try to win enough primaries to keep Dean from winning on the first ballot. Then they at least have a chance to make some deals on the convention floor and select a compromise candidate. Unless some big event happens to change the face of the campaign (for example, Kerry or Edwards or Lieberman dropping out) I see Dean winning the nomination fairly easily.