Like Noam, I find the possibilty of an Obama-Clinton ticket to be remote--and, for reasons widely discussed here and elsewhere, fairly unappealing on several levels.
Still, I think it's worth remembering what's important about the vice presidency. At the risk of sounding like a fuddy duddy, by far the most important criteria for the job should be the ability to serve as president--and, more specifically, to take over in a time of crisis, since by definition a president failing to serve out his or her term constitutes some sort of national crisis. Assets like experience should count even more; so should an established, unambiguous record on major issues of the day. (My model for the best vice presidential pick of recent times, Al Gore, fits this bill perfectly.)
Say what you will about Clinton, but I am confident she meets that standard. I am not nearly so confident about a lot of other names I hear frequently mentioned in this discussion. People like James Webb, Kathleen Sebelius, etc. may be extremely impressive and appealing politically, but I simply don't feel they've gained that trust. At least not yet.
That doesn't mean Obama should pick Clinton. Or that you can't find a vice president who is both qualified and politically interesting. (Ted Strickland maybe?) But it does mean that some of the less exciting names you hear in this discussion (Joseph Biden, Tom Daschle) and maybe even some you don't (George Mitchell, Jack Reed) deserve more attention than they've been getting.