Pennsylvania hasn't voted for a GOP Presidential candidate since 1988. With the exception of only two states (Maine and Nebraska), all states have a "winner takes all" rule in giving its Presidential Electors to the person who gets the most votes in his state. For example, Al Gore won the state of Wisconsin by approximately 5,000 votes in 2000 but was granted all ten Electors from the state instead of five or six he may have gotten for splitting the vote.
Article II Section I of the Constitution gives each state broad latitude in how it chooses its Electors, so Pennsylvania's prospective change will almost certainly survive any court challenge (if it comes to that). Right now there are eighteen Electoral votes in Pennsylvania (2 Senate seats and 16 House seats) and what will probably happen under a new plan will be each candidate will be awarded all the congressional districts he carries, and, if he gets the most votes in the entire state, he'll get the two other Senate Electoral votes as well. This has huge implications: even if Obama were to carry the state in 2012 it is very likely he would lose at least ten of the congressional districts which would grant the GOP candidate ten Electoral Votes. Obama's will be overwhelmingly strong in Philadelphia and Pittsburgh but he'd lose in most of the rest of the state which would account for his actually winning less Electoral votes than his GOP opponent.
If you assume 206 Electoral votes are certain for the GOP and 60 more are leaning in the GOP's direction, they would only need four more to win the needed 270-they would come from Pennsylvania.