Governors Tim Kaine, Evan Bayh, and Senator Joe Biden seem to be leading the field according to the Washington Post. If Kaine gets it, I would have seen the first time they stumped together at T.C.Williams High School in Alexandria. Post available here.
Kathleen Sibelius is also mentioned. Chris Dodd, Hillary Clinton, Sam Nunn, Chuck Hagel and Jack Reed are mentioned as long shots.