OK, it was not molded, and very easy to disassemble. I have some pics on flickr:http://www.flickr.com/photos/tastewar/sets/72157632466799667/
I checked connectivity on each of the four wires, which seemed good. I checked connectivity from each USB contact to its connection on the board, and all of those seemed good. Looking at the rest of the soldering, it all looked OK to me, so I am at a loss now. I thought perhaps one of the wires might have been shorted to ground while scrunched up inside the connector, but testing the cable without the shell, and clearly no shorts, showed the same failure.
I guess I don't really need a replacement -- the cable was just part of the larger kit, and I didn't buy the kit to get the cable, it just would have been a nice addition. And I can use an FTDI cable (actually, a Modern Devices USB BUB II) in its stead. I was really just trying to test the serial port of the RPi for potential other use anyway. Just took a lot longer to prove that the serial port was OK with a bad cable