I hadn't thought of this, but it makes sense. Jackie Calmes reports for the WSJ: "Some in the Clinton camp also noted a possible problem for a party-unity ticket: Bill Clinton may balk at releasing records of his business dealings and big donors to his presidential library." You have to assume the Obama campaign wouldn't agree to an Obama-Clinton without without at least their own vetting team getting a crack at those books. And it seems very unlikely that Obama would want to fight Bill's battle for non-disclosure to the public.
Matthew Yglesias is a former writer and editor at The Atlantic.