According to a press release I just received (emphasis mine):

Common Cause’s election reform team will monitor voting problems and concerns that may arise tomorrow during the Democratic presidential primary in Pennsylvania, where an unprecedented turnout is expected, including a huge surge of new voters.

Some 7 million Pennsylvania voters are expected to vote tomorrow on paperless electronic voting machines that lack the ability to do a recount. Common Cause will help monitor problems reported to the Election Protection Coalition’s national voter hotline, 1-866-OUR-VOTE.

Pennsylvania election officials are bracing for unprecedented turnout in a state with a recent history of voting machine problems, and where voter registration and registration changes have surged in recent months.

How will PA affect the big picture? Not very much, according to PoliBlog:

I must confess, it is difficult to get too excited about the Pennsylvania primary, given that no matter the result, we will be in basically the same position: Obama with more popular votes and pledged delegates, and therefore on the surer footing for the nomination.

Another part of the big picture concerns close elections. As Florida 2000 and Ohio 2004 taught, close elections strain the legitimacy of rules otherwise considered minutiae. Will voting equipment and voter rolls join Michigan and Florida as flash points in the Clinton-Obama saga?