Tom DeFrank has something I hadn't heard before:
A top Democratic source with insight into Bill's and Hillary's states of mind says the Clintons are convinced that a Democratic presidency is all but certain no matter how messy the fight for the nomination.
In that scenario - which the Obama side and some Democratic elders worry is wishful thinking at best, delusional at worst - there's no downside for Hillary doing whatever it takes for as long as it takes.
My earlier understanding was that Hillary refused to quit because she believed Obama would lose to McCain. I didn't see things the same way, but there was a certain nobility to her stubbornness (even if the logic was self-serving). If DeFrank is right and I'm understanding him correctly, Hillary now thinks both she and Obama would win and just wants to claim the prize for herself. It's, uh, not exactly the most principled argument for denying the nomination to the guy who's basically won it.