NBC's Ron Allen makes a smart point I haven't seen elsewhere: Hillary Clinton's two main campaign justifications contradict each other. Justification number one is that Obama can't win "the big states that Democrats will need in November." She claims this because she's winning those states in the primary, and she's doing it because she's winning a slightly larger share of the Democratic electorate there. Justification number two is that the extended primary isn't hurting the party's chances, because the Democratic base is bound to unify in the fall. As Allen points out, if that's true, then Obama shoudn't have a problem winning those states in November.
In general, I think the coverage of Pennsylvania is wildly overblown. What happens tonight is not going to effect the outcome of the nomination. Obama will be the nominee, and the only thing that could stop him would be a massive scandal. If Wright and Bittergate couldn't dent his standing, a loss in Pennsylvania won't, either. The only thing the Pennsylvania results could possibly change is the timing of Clinton's departure, and even that won't happen unless Obama somehow pulls off a shocker upset win.
The conventions and structural biases of journalism dictate that importance must be read into whatever outcome occurs, but the fact is, it really doesn't matter.