From what I've been told, once you're accepted you get to rank the campuses in order of your preference. Then they do some sort of matching thing, but I think it's first come, first served. I've been told that the Chicago campus usually fill up sometime around January or February, but that Peoria and Rockford are open longer.
Oh, and if you go to Peoria or Rockford, you do M1 down here in lovely Chambana! Some people do all 4 years here, but they are mostly MSTP.
I am an undergrad at UIC. Last year one of my friends ( she is an URM, though), a 2001 aug. mcater, got an interview invite from UIC in late Feb. and she got in the chicago campus. Don't worry about it, you will be fine.