Why would John Edwards drop out of the presidential race before February 5th? He's already on all of the ballots. He has plenty of national name recognitition. It's only 5 more days. No matter how broke Edwards campaign is (and he claimed recently that it had a few more million dollars in the bank), he could keep it running for 5 days. Even without any ad-buys, Edwards could pick up 10-15% of the delegates on 2/5, which would give himself the kingmaker role he seemed to want.
Some have suggested that Edwards won't endorse because he will want to play a role in a populist shift in the Democratic party, and that by siding with one candidate over the other he loses that opportunity. If that is the case, however, why wouldn't Edwards stay in the race, participate in tomorrow night's debate, and remain part of the national conversation? Quitting now robs him of whatever spotlight he had left.
The only reason Edwards would drop out is if he thought he could achieve more by endorsing a candidate BEFORE 2/5. His endorsement will mean far less after that. Expect an Edwards endorsement, likely of Obama, in the next few days.