More discussion on this story from the Wall Street Journal (via BooMan):
Obama: “His focus now includes five colleagues in the U.S. Senate — Joseph Biden, Evan Bayh, Chris Dodd, Hillary Clinton and Jack Reed — and two governors, Tim Kaine of Virginia and Kathleen Sebelius of Kansas, according to Democratic operatives, though he could still make a different pick.”
It can't be a senator. I've blogged about why, so I won't repeat the whole rap, but suffice to say, the Senate Dems need to remain in the Senate. And, of course, Senators Bayh and Clinton would pretty much destroy the party base as well as obliterating Senator Obama's change theme.
Therefore, if it's any name from this list, it'll be either Governor Kaine or Sebelius. And if I had to choose, I'd go with Governor Sebelius.