I'll echo Dana and Ann's comments about Kathleen Sebelius and add a few of my own. As this nice New York Times piece points out, Sebelius reinforces Obama's core message in a way none of the other potential VPs do. She has a record of working with Republicans to achieve progressive aims -- increasing school funding, blocking the construction of new coal plants -- in exactly the same way Obama promises to do.
That message of bipartisan change, which has been the center of Obama's brand since his convention speech in 2004, has gotten lost in the shuffle over the last month as Obama has dived into the general election. It's still his strongest argument and I can't think of a better way to reinforce it than picking Sebelius. Picking Biden, on the other hand, would signal a deliberate decision to abandon that approach and run a more traditional Democratic campaign, and if there's one thing Obama isn't, it's traditional.
What's more, Sebelius' relative absence from VP speculation over the last few weeks actually makes me think she might just be the pick. After all, Obama will almost certainly get a bigger bump if he picks someone surprising than if he chooses someone well-known.
--Sam Boyd