Feingold says no,

The big problem will picking a candidate to support four years away from an election is;

I want you to know that I've decided to continue my role as Wisconsin's Junior Senator in the U.S. Senate and not to seek the Democratic nomination for President in 2008.

(full letter)

I was kind of expecting it. In a way I am glad that he is not going to run. I think it would of been half hearted, since he basically endorsed Obama, and I think a half hearted run would hurt the progressive movement more than it would help it. I think more of these letters are going to come out in the next month (including one from Hillary) to make room for the big democrat star Barack Obama. Although I will wait to see who is actually going to run before I pick my next preferred candidate.