As with many departments, even if you don't hear anything by the deadline, you probably will hear something on or immediately following. Given the backlog of people accepting right until the deadline, departments shift around their spots and move people off the waitlist. I'm almost positive Pitt did this last year (after I declined my offer).
It was my experience that some programs even pulled people off the "rejection" list and offered them admission with full funding.
The short of it is, although the "official" deadline is the 15th, there are many unofficial things that get done to ensure that a program's cohort size will be sufficient.
Good luck!!!