Hi Kim! I'm really glad you brought this up - I'm the creator of the UBW, and I get asked this question a lot.
I think the way that SparkFun probably should have phrased that text would be "could be used as a USB to UART converter board, with the proper software". See, I haven't gotten around to writing the code necessary for the UBW to act like a USB to UART converter, yet. The latest software version is 1.4.2, and it does not have that functionality built in. The next version (1.5.0) most likely will have that feature built into it, I just haven't quite gotten to writing that part of 1.5.0 yet. Soon, hopefully. Probably Q1 next year. Along with a ton of other neat features.
However, if you or somebody you know wants to write that code, it is just straight C code on the PIC. All of the development tools are free, all of the project files are up at the UBW website, and anyone can add to it. (It's really not very hard.)
Currently the UBW brings a 'serial port' interface into the PIC from the PC. All that's left is to 'connect' up that serial port interface (in the UBW firmware) to the USART hardware which exists on the PIC, and then you could use it. There are a few minor details - like intercepting baud rate change messages, and being able to send back handshaking status to the PC, but those are not hard functions to write.
Please let me know if you have any questions. The Arduino's hardware, in concept, is almost exactly the same as the UBWs. But the purpose of the two devices is pretty different, and the supporting software reflects this. I have an Arduino and I love it.
If you have more general questions about the UBW, or want to research more about what can and can't be done with it, check out the USB forum up at SparkFun, where we've been chatting about the UBW ever since it first came out a year and a half ago.