Two counties to watch in particular, per MSNBC's Chuck Todd: Lancaster and York. These are, respectively, the Phildelphia exurbs, and the very exurbs. The demography nerds tell us that upscale, educated social moderates are creeping out into these areas, which make them more likely to be Obama country than they would have been, say, six years ago, when Ed Rendell and Bob Casey matched up for the state's last high-profile Democratic primary.
As of now (about 9:45), it looks like York is split 50-50, with about 8 percent of districts reporting, and Lancaster is about 57-43 for Obama, with 14 percent of districts reporting. For what it's worth...
Update (10:03): Chuck Todd just said York should tilt to Clinton (though he also said no results are in there, which isn't true). If true, that 50-50 margin has to be encouraging for Obama.
Update 2 (10:05): A new York number (as opposed to a New York number): Hillary up 54-45 with 31 percent of districts reporting...
Update 3 (10:07): A new Lancaster number: Obama up 53-47 with 30 percent of districts reporting.
Update 4 (10:11): It looks like Rendell won Lancaster back in 2002, but Casey won York.
Update 5 (10:21): Rendell won Lancaster 59-40 in 2002.
Update 6 (10:31): Obama still at 53-47 in Lancaster, with 42 percent of districts reporting. Clinton up 55-45 in York with 62 percent of districts reporting.