Definitely contact UIC. I and many others received the official acceptance letter several weeks ago. There's nothing in it that's terribly urgent, so don't stress about it. The letter was probably just lost enroute.
You should join the facebook group for the Class of 2012. That's where the most communication among us seems to be happening these days. There's not a whole lot going on, but it's more UIC talk than you'll find here.