For my fellow waitlistees, from penn's waitlist FAQ-- I thought these parts were especially helpful:
"The list ranges from a larger number (400) in March to a reduced size (100) later in the year (May). Reductions occur by self-selection or Committee action."
"Acceptances from the waitlist may be made at any time. Generally, waitlist acceptances are not made until May. Acceptances may continue, on a limited basis, throughout the summer until the first day of class, August 12, 2013."
and
"The number [accepted from waitlist] varies from year to year, depending on the number of people who withdraw from the initial acceptance group. We are not able to predict a specific number at this time. Best guess: 12 to 25."
Congrats again to everyone accepted, and to all of us for making it this far! 🙂