Step by step testing inside a Linux Terminal usage for the USB Arduino Diecimila Board...
Issued as completely Public Domain by B.Walker, G0LCU, Circa 30-03-2011.

1) Boot up to your Linux flavour as root, (or run in a root Terminal).
2) Temporarily disable any net access or use a stand-alone machine.
3) Plug in the USB Arduino board.
4) Open up a, (root), Terminal and clear the screen.
5) clear<RETURN/ENTER>

6) Check that you have a device as ttyUSB?
7) ls /dev<RETURN/ENTER>
NOTE:- ? above will probably be 0.
8) If ttyUSB0, (to 7), appears then carry on; otherwise ignore the remainder of this text.

NOTE:- Assuming the device is ttyUSB0 from now on, change below if needed to your ttyUSBx device.
9) Place a wire link between ANALOG 0 and the 3.3 V terminals.
NOTE:- 'chmod' is not really needed but added for fullness!
10) chmod 666 /dev/ttyUSB0<RETURN/ENTER>

11) Set up the on board serial I/O to 1200bps and RAW data transfer.
12) stty -F /dev/ttyUSB0 1200<RETURN/ENTER>
13) stty -F /dev/ttyUSB0 raw<RETURN/ENTER>

14) Now start printing characters to the screen...
15) cat < /dev/ttyUSB0<RETURN/ENTER>
NOTE:- The characters will be from about 168 to 174 decimal allowing for bit error of the Arduino ADC.

16) Remove and replace the wire link from ANALOG 0 and GND or ANALOG 0 and 5 V too.
17) Press 'Ctrl-C' to stop any characters being printed to the Terminal.
18) Unplug the USB Arduino board.
19) Power down the computer IF REQUIRED.

Enjoy finding simple solutions to often very difficult problems... ;o)