Clear WiMax is offering a cheap $25 data plan that works only with Apple iOS devices. They sell you a device, and it acts as a WiFi hotspot only for Apple hardware. "How does it know what kind of device I'm using?" is the first question that popped into my mind, because there is nothing in the WiFi protocol that requires a device to identify itself. Well it turns out the hotspot knows which blocks of MAC addresses are issued by Apple. MAC addresses are easily spoofed though, as per the link below. I am considering getting this, I wonder if its available in Baltimore.
Update: Engadget reports that hackers have figured out how to modify the settings on the device to prevent it from MAC filtering!