The Best Western on 22nd Street near the Parkway has rooms available. They have over a 100 rooms available (last time we checked). They offer a U Penn rate of $85 per night. Workshop attendees can just call in, make the reservation and ask for the UPenn rate. This hotel is about 2.5 miles from the conference location. More details about the hotel can be found from the hotel website.
There are many events in Philly around the dates of the workshop, so hotels will be filling up quickly. We urge you to make reservations in the above hotel or others listed below as soon as possible.
The Bank Street Youth Hostel is located on Bank Street between 2nd and 3rd street. Thanks to Nir Halman for pointing it out. The rates are really cheap (approx. $20 per night). The conference site is accessible from the hostel by the blue line subway.
The Sheraton University City Hotel and The Hilton Inn at Penn are the only hotels within walking distance of the campus.
Here is a listing of all the hotels that offer special UPenn rates.
Here is a link to a listing of a variety of hotels in the area (use the "Center City Philadelphia Hotels" link for hotels near the university). However, most of these are not within walking distance, but are accessible by public transportation.
Here is some general information about the university and directions to get to the workshop.