It's not unheard of to get lost in the shuffle. When I got hired they left me hanging for a few weeks. When they finally did call, they said, "I'm sorry, but we are gonna have to push your class back a few weeks, because it's full." I said, "so does that mean I'm hired then?" She said oh yea, they didn't tell you already?
So I guess the moral of the story is, papers get lost, and the communication between everyone there isn't perfect. Give them a call.