Maureen Dowd's column today -- in her usual snarky style -- hit on a critical point in the discussion about Clinton "earning the right" to the #2 slot, as some would have it.
The point is, Obama can reasonably say that, to be considered for the #2 position, he needs to be satisfied that there is nothing lurking, Thomas Eagleton-style, in Hillary or Bill's background. Clinton has often made the claim on the stump that she has been through the most thorough possible "vetting" in the multiple, abusive special prosecutor investigations unleashed on the Clintons in their 8 years. But there is a lot more water under the bridge in the 8 following years, and lots of stuff that happened toward the end of the Clinton administration that was not followed up on when Bill Clinton was a private citizen ex pres, but could well be dug into if he were the second spouse.
At the very least, Obama can say, to be considered for the VP slot, we need your unambiguous endorsement, the end of your campaign, and all financial records of both of your Senate campaigns and of your presidential primary campaign for the purpose of vetting you. Also, we need all records of Bill Clinton's foundation and library, and of his speaking engagements since leaving the White House.
At that point, either Clinton will pony up, or she'll think twice about it. If the latter, she can reasonably be portrayed as not being willing to do what it would take to be considered, and that her withdrawal is her own choice. If she give the records and endorsement, Obama can and should consider whether she would bring unity to the ticket and the balance that he needs. If he decides against her, at least he would have given her a fair shot. But by that time, she will have to have given her endorsement and essentially all the opposition research material Obama could ever want if she went back on it.