Since the results came into focus last night, I've heard a couple explanations for why Hillary might stay in the race even though she has no chance of victory. Most of them boil down to proving some sort of point in West Virginia and Kentucky. For example, Tim Russert (I think) suggested that winning big in those states lays the groundwork for a kind of "I told you so" argument for Hillary in 2012 if Obama's loses to McCain.
This morning, Marc Ambinder laid out a variation on that theme:
At the highest levels of the Obama campaign, there is no appetite for any talk of a unity ticket so far. Still, big victories in West Virginia and Kentucky will help Clinton make the argument that she is indispensable.