Maybe the answer to this should be obvious, but I'm just wondering if the Arduino Duemilanove board can be used as a regular usb-serial converter? i.e. Can I connect a serial device to the Tx, Rx, +5v and Gnd connectors, to communicate with a PC?
If your device has TTL tx/rx lines, then there should not be a problem. The pc will just see the device the same as it sees the chip on the arduino. Load a non serial script into the arduino so it won't interfer with any of the communications between the pc and the device.