Barring anything shocking happening, Pennsylvania will matter quite a bit this cycle. Shocking!
As Chris Bowers notes, with Wyoming and Misssissippi being highly probable net delegate wins for Obama, the pledged delegate math by the Ides of March is back to where it was Monday morning --- Obama up by a significant margin but with far fewer delegates left to contest. Pennsylvania right now is the only chance Clinton has a blow-out win that has enough delegates to matter. So let's take a quick look at the Pennsylvania Democrats.
1) Pennsylvania is a closed primary state, and it is a hard closure. Non-affiliated/unaligned/independent and registered Republicans voters do not vote in the Democratic Primary.
2) Pennsylvania Democrats are on the whole ideologically about the median national Democrat, at least measured by our House delegation.
3) 'Pittsburgh on end end, Philadelphia on the other, and Alabama in the middle' or Pennsyltucky are two quick and simple ways of describing the state politically as a whole. The rural/mountain regions tend to vote conservative to very conservative while the cities vote liberal and come out in large numbers.
4) Delegate allocation rules can be found at the Greenpapers.
5) Delegate allocation rules significantly favor Obama --- very plausible for him to lose the state by eight to ten points and win the delegate fight.
6) GOTV operations are not that good in SW Pa. The machine Democrats have been fighting a long and slowly losing battle in Allegheny County (Pittsburgh's home county) with the reformer Democrats as the reformers are able to out-hustle the GOTV on lower value races. Endorsements have not been delivering the same type of punch as they used to.
7) In 2006, most regional GOTV was provided by outside organizations; the Casey-Rendell coordinated campaign just sought to flush the Dem supermajority precincts and call it a day.