The commuter bus service is called CoachUSA (service provided through Wisconsin Coach Lines) and their website is at http://www.coachusa.com/  www.coachusa.com

It appears that there has been a change (or will be a change as of Feb. 21) wherein this bus service is being provided only to/from O'Hare, and no longer serves Midway. A note on the website says: "Effective Monday, February 22, 2010 Wisconsin Coach Lines will no longer service Midway Airport."

The service from O'Hare runs daily every hour starting at 6:20 AM (no bus at 7:20 AM, however) from O'Hare to Milwaukee and 8:05 AM (no bus at 7:05 AM, however) from Milwaukee to O'Hare. Prices given are $26.00 one-way or $47.00 round-trip. It appears that you can buy tickets online within 4 months of your scheduled trip, so people can start buying online soon (like next week).