I am also on the waitlist/alternate list at Pitt. I spoke with Rose Mangold a while back and she said that the deadline for those offered a spot on the waitlist to send in their 'letters of intent' was March 10. After all of the letters are in the admissions committee would meet again and rank those from whom they'd received letters. Pitt, however, will not inform you of your position on the list and you must simply wait it out...