Saw a video posted here a while back. There is a magnet inside the case that trips a reed switch sensor on the Mac. I can't recall, but it's on the back, near the connections. Should be easy to find by taking the case off, and probing around the inside with a small metal object to locate the magnet.
The case helps direct the airflow, and if anything comes into thermal contact with it, it can also be used as a heat sink in it's own right. Without it, you'll run a real risk of insufficient cooling, causing component failure.