Common Solutions Group: StoneSoup.org
For the adventurous, curious, or penurious, it is possible to make it from Philadelphia to Princeton via public transport.
For those who prefer trains, NJ Transit also runs service from Newark Airport to Princeton Junction with a short connecting ride to the university, about 4-5 blocks from the hotel.
PHILADELPHIA AIRPORT --> PRINCETON UNIVERSITY
Take the SEPTA R1 line from the Airport:
http://www.septa.org/schedule.cgi?route=r1s&day=1&return=on
Transfer at 30th Street (Amtrak) Station to the R7 and take that to TRENTON
http://www.septa.org/schedule.cgi?route=r7s&day=1&return=on
From TRENTON Take the NJ Transit train to PRINCETON JUNCTION & transfer to "the Dinky" to get to Princeton
http://www.nj.com/njtransit/nec_e_mf.html (shows the connecting 30th Street Station trains as well)
NEWARK AIRPORT --> PRINCETON UNIVERSITY via NJ Transit
http://www.nj.com/njtransit/nec_w_mf.html (includes "Dinky" schedule)