OK, it's awfully early to talk this, but there's an interesting story brewing here: It will be the first time in I'm not sure how long that there wasn't either a sitting president or VP in the race.
Most of the names that get mentioned are people in the senate, but that's a mistake. Senators don't get elected president. JFK did it, but I'm not sure there was another 20th century example.
My comments on specific dems below the fold.
First, let's consider the lot that ran this year. We can throw out the longshot candidates: CMB, Sharpton, Kucinich. They might run, but they won't make it out of the primary.
Lieberman and Graham didn't make much of a dent. They almost belong in the preceding paragraph.
Kerry is damaged goods. He might think he can do better in '08, but he probably won't get the chance (nor should he).
Gephardt was past his day in 2004. 2008 will be even farther past.
Edwards might have a shot if he can do something in the next 4 years to keep himself in the public eye. More likely, like Dan Quayle, he'll disappear from the spotlight.
Dean is also damaged goods. His Yahoo ads are hilarious, but reinforce the idea that Dean is a joke.
Clark seems the best chance out of the lot to make a comeback in 2008. Getting whatever necessary assistance so he can crank out an instabook a year on key foreign policy and anti-terrorism policies would be very helpful. He should be working right now on his "how to get us out of Iraq" book immediately.
In the Senate/House, there's not much to get excited about. No one really wants Hilary to run nor thinks that she can win if she does. Obama should run, but not in 2008. I think that he has the chance to win from the Senate, like JFK, by virtue of being the first black president (in JFK's case it was first Catholic). Maybe Diane Feinstein, but she seems a bit of a longshot.
Governors? The only one who really gets me excited right now is Bill Richardson (NM). He would almost certainly deliver a landslide for the dems among Latino voters, which will make all the difference in 2008. Warner doesn't do much for me, and I don't think he'll deliver VA let alone any other swing constituencies.