When this happened to my Verizon XV6800 and I was shown how to force it into boot loader mode (
http://forum.ppcgeeks.com/showthread.php?t=15987) it did show Serial on the bottom of that tri color screen and this changed to USB when connected to the PC. Active Sync on the PC showed no connection at this time.
I used the ROMUpdateUtility.exe 3.16.4.3 that came with DCD's Kitchen. After acknowledging the first two screens it put up the box saying it was verifying the information on my phone then presented me with the Update option. Continuing from there it gave me the typical From: Image Version and To: Image Version. Hitting Next loaded the .nbh file I had in the directory I started the ROMUpdateUtility from.
You should have only one .nbh file in that directory and I used the stock VZW ROM when I did mine.
After this I have run up several ROMS using the ROMUpdateUtility and on a couple of those occasions I have had a message in Windows saying an unknown USB device was connected.
I have played with this again a few times tonight and found I will get an ERROR[264]: CONNECTION if I have canceled a download and try it again without re-entering bootloader mode.
Hope that helps. If you are still having problems post some details about the loader you are trying to use and such.
Ks.