I called Penn yesterday and found out some info about the wait list.
Basically, there are about 500 people on it. Of these 500, they expect to call about 35-50. This was a little garbled because he kept reiterating the 10% number (which would obviously be 50). He said that the acceptance rate off the WL is about the same as the overall acceptance rate, which didn't seem germane to me, but whatever.
He also said that they will be making the next round of offers in about 2 weeks.
So, I guess I'll keep hoping, even though I signed my DC apartment lease today...