Webb is well known to have women problems. He has been characterized (in his 2006 race) as insensitive to women. (Supposedly he was patronizing to woman officers below him and was disinclined to promote women. He argued during his campaign that the opposite was true. All in all, the last thing Obama needs.) This will rear its ugly head again and make it MORE difficult to get Hillary voters.
I think any OTHER woman would also make it more difficult to get Hillary voters and Sebelius adds nothing to the ticket. If Hillary and Obama would convincingly announce that she was offered VP but declined, that might help; on the other Hand why would Hillary admit to her own supporters that she turned down the chance to be the first woman VP? The only way I can see this happen is if she is offered the majority leader post and makes a convincing argument that she can advance her agenda better as leader of the Senate. Harry Reid and Chris Dodd (possibly also Dick Durbin) might not go along with this.
All other things being equal, Clark (military cred), Strickland (Ohio cred) or Rendell (street cred and great campaigner - but sometimes goes off message) seem Obama's logical choices if not Hillary. Longshot pick: Bob Graham. Former chairman of Senate Intelligence Committee, beloved in Florida (even more than Jeb), not too old but reeks gravitas. Downside: sort of boring, but this may actually be a benefit to Obama. Bonus upside: nobody has mentioned him in all the veep speculation - makes media look stupid. Had Gore selected Graham instead of Lieberman, we could have avoided the last 8 years of misery. Lieberman was selected to appeal to Jewish voters in Florida; Graham would have appealed to ALL voters in Florida. Bonus longshot: Schweitzer of Montana, depending on how he polls in CO, NV, NM, WA, OR.