Chris has an excellent post up about the Clinton campaign's dwelling on how pledged delegates aren't locked into a particular candidate. I'd just add one thing: The person responsible for this procedural quirk is none other than Harold Ickes, the wily Clinton operative currently overseeing her delegate-whipping effort.
Pledged delegates actually were bound to their candidate through the 1980 campaign. That was the year Ickes' boss, Ted Kennedy, entered the Democratic convention needing hundreds of pledged delegates to abandon Jimmy Carter, who'd already clinched the nomination, in order to have a shot at it himself. Ickes' strategy was to first engineer the procedural change that would allow delegates to switch their allegiances, then to pick off said delegates. It was a tall order, and Ickes obviously came up short. But, from what I'm told was a kind of consolation to the Kennedy forces, the rule was changed before the next presidential cycle, which is where we are today.
Why am I recounting this bit of history? Two reasons: 1.) Because I can--it was just lying around in my notes. And 2.) because, while I 90-percent agree with Chris and Josh Marshall that the Clintons' goal here is to create a "fog of nonsense" (terrific term, by the way), I think Ickes himself is just crazy enough to want to target Obama's pledged delegates, the same way he pursued Carter's in 1980, and against much, much longer odds. If nothing else, Ickes probably figures he can extract some concessions the way he did back then.