I have used this cable for other similar devices which provide a serial port this way (3.3V only, I hear there are 5V ones too). This cable includes the necessary 3232 type circuit for converting between the 3.3V on the device side to 12V on the RS232 side. This is where I got the idea: http://www.nslu2-linux.org/wiki/HowTo/AddASerialPort
NOKIA DKU-5 USB Cable
It's < $5 USD. After cutting off the cell phone plug on the end the wires worked out like this:1
3 - orange, not needed
4 - green, 3.3v from device
6 - white, serial RX, data to device
7 - blue, serial TX, data from device
8 - black, GND This particular one requires power from the device. There are other similar cables which draw their power from the USB host instead. I also have some other DKU5 cables which are RS232 on the host end, not USB, and you can find tons of those on Ebay as well. It is good enough for a console but probably too flaky for much else without connecting the flow control pins. I plan to use one of those for Raspberry Pi.