Jonathan Stein in Mother Jones makes the argument that Obama and Edwards are very close reform candidates drawing much the same people. After Iowa and/or New Hampshire should the lesser candidate withdraw in favor of the other? This argument is also starting to be made elsewhere aimed at Edwards.
By the rules of Democratic convention delegate allocation there is little incentive for a candidate to withdraw as long as a candidate is still getting 15% of the vote in many areas.
Edwards has an advantage of being the leading second choice candidate in states with caucuses. As candidates don't make the 15% threshold their supporters will migrate to Edwards.
I don't think you will see any major candidate withdrawals until after Iowa and then Biden, Dodd, and Gravel may withdraw. Kucinich will not withdraw. He is in it just to provide a more liberal alternative and move the party leftward. Richardson looks to be out of the race early.
If Edwards or Obama doesn't get 15% in New Hampshire then there is a good case to be made for a withdrawal.
Will that candidate endorse the other to stop Hillary? It is impossible to say right now. Hillary is immeasurably better for most Democrats than any Republican running. Will Edwards or Obama want to anger an incoming Democratic administration? Only if there is a realistic chance of stopping Hillary will a withdrawal and endorsement of another reform candidate occur.