Tim,
I tried this on my end by modifying the usb_dev_bulk example (.c file attached). The only difference I can see between your code and my code is that I used USBDCDTerm(0); instead of USBBulkTerm(o);. I would recommend doing this. I'm not sure where your code was added as this can also make a difference. I was able to see my LaunchPad come up as a Stellaris Device Firmware Upgrade device in the Device Manager and was able to reprogram it with a different binary file using dfuprog.exe (instructions on how to use this and where to find it can be found in the readme.txt in the boot_demo_usb project folder for the dk-tm4c129x). Make sure to program your binary to the board using the ICDI Debug USB port and then switch power to power from the Device USB port on your LaunchPad, as to not confuse the number of DFU devices found.
(Please visit the site to view this file)
-Rebecca