The D-Bay port is for the D-Bay module. You connect this module ( looks like a very large USB card reader ) to the port. This module can take additional drive modules. I think there is a port replicator for it also. It is kind of pointless since you can get drives and stuff for either FireWire or USB2 cheaper.
I was curious the first time I saw that port myself. But it is indeed a special USB+Power port. I've had them on HP Point of Sale Machines I've installed. They carry either 12V or 24V to run receipt printers or other external peripherals requiring more than USB's standard 5 Volts.