John Edwards has dropped out (and I don't understand why he picked now rather than waiting a week until after Super Tuesday; I would have preferred to have a day to just exult in Rudy Giuliani's epic collapse), after I already sent in my absentee ballot for him for the New York primary. Looking at the three presidential primaries I have voted in, this brings my streak to 0 for 3 on voting for a candidate who is still in the race at the time of my primary. However, the streak should end in 2012, when I vote for the incumbent Democratic president, who will likely be unopposed.
Now that it's down to Obama and Clinton, I'm not endorsing another candidate in the primary; I look forward to supporting Clinton and/or Obama on the Democratic ticket. They're both strong candidates and I don't want to get involved in intraparty squabbles right now; I'd rather focus on beating the Republicans.
If you were an Edwards supporter and are now totally undecided between Clinton and Obama (and only if you don't have a preference), my endorsement is to vote for Edwards anyway. This way, Edwards may still get some more delegates, and in the unlikely event that neither Obama nor Clinton has a clear majority of delegates, the Edwards delegates will have a voice at the convention. (And in the more likely event that one of them wins it outright, that's fine, because you didn't have a preference anyway.)
Another option is just to split your vote based on which delegates you like and trust, insofar as you recognize their names. On the NYC ballot, many of the Clinton delegates were household names (ok, household names for NYC political junkies: current and former City Councilmembers, state legislators, and such), while I hadn't heard of most of the Obama or Edwards delegates. This is presumably because Clinton is the home-state senator, so the local politicians have loyalties there. I assume the situation is reversed in Illinois and North Carolina, and more mixed up in other states.