Grad schools tend to be very professional regarding telling people they did, or didn't get in (either way, you should get an e-mail or some sort of notification).
It's possible something got screwed up. Maybe you missed it, maybe you got it, but either way just call admissions.
I'm in a similar situation for DMU. It's been over 3 weeks, almost 1 month but haven't heard from them.
I called and the admissions staff said to wait until next Tuesday since the Dean hasn't made the decision yet.. X( It's more waiting for me.. Definitely call them-.
They're probably looking at all the late-cycle applicants and deciding who is the best. By lengthening your wait time they can view more applicants and make sure they are capping off their classes with the best possible candidates.
In other words, it means you were not an easy acceptance. This happens at the end of every application cycle.