Nader submitted signatures to be on the PA ballot. According to a recent newspaper article, there was no challenge of the signatures before the deadline, so they are accepted and he is on the ballot.
In 2004, the PA. Dem Party successfully challenged Nader's signatures. Sufficient numbers were found to be invalid that he was removed from the ballot. However, that was tainted by the charge that public employees and public dollars were used in the challenge.
http://www.timesleader.com/news/With_ease__Nader_on_Pa__ballot_08-08-2008.html