I'm in Kuwait at the moment and the camp where I am has pre-paid wireless hotspots that use MAC filtering to block unauthenticated users and redirect them to a login page. After, successful authentication the MAC is white listed. It's pretty weak security that can be bypassed by:

1. Running Kismet or Airsnort to capture some packets
2. Running Wireshark to find traffic other than people getting stuck at a login page (i.e., authenticated users)
3. Changing the MAC of your NIC to that of an authenticated user and connecting as usual

P.S. I'm in Kuwait now killing time before we move to Iraq.