Here's an old USB Debug Port presentation that may be useful to those new to the concept:
http://www.usb.org/developers/presentations/pres0602/john_keys.pdf
Is the NET20DC really the only USB Debug Device available? I tried searching for others, but didn't find any.
I think the design might become easier if you just create the debug part of the device and use the serial side of a plain usbserial device as direct interface.
Actually I suggested something along those lines already in the wiki page. :)
Well it looks like in the pdf presentation above they have a Serial->USB debugger. It looks like a development board though because there are a bunch of unnecessary stuff on it. I'm thinking a Serial->USB debugger is the way to go to keep the costs down, and it gives the terminal PC (serial end) more flexability. If one wanted to have it USB on both ends they could always use a USB->Serial adapter for the terminal PC (serial end). Also, this way there would be no need for any kind of a special driver for windows/linux, it would just show up as a serial communications device. Any suggestions, questions, comments?