Ugh. My letter said that they should know by April 15th. The DGS told me that they over-admit, so they have to wait for a certain percentage to decline offers before they move to waitlisted candidates. On top of that, this department admits from the waiting list according to area of specialization. So for example, if you're a medievalist, you have to wait for a number of medievalists to decline before you'll get considered (a bunch of Americanists turning down offers wouldn't help you).
Don't know if that helps...maybe I'm just whining because it sounds like it doesn't bode well for me personally.