Connect the Arduino to a usb port, open a terminal window and type
What messages do you get, if any, for the usb device?
For example, on my pc I get
[22146.520012] usb 5-2: new full speed USB device using uhci_hcd and address 5
[22146.722901] usb 5-2: configuration #1 chosen from 1 choice
[22146.727106] ftdi_sio 5-2:1.0: FTDI USB Serial Device converter detected
[22146.727738] ftdi_sio: Detected FT232RL
[22146.728513] usb 5-2: FTDI USB Serial Device converter now attached to ttyUSB1
I know that on earlier versions of Ubuntu, a bug in the brltty package forced the arduino to be recognized as a braille device. I think it was fixed before Ubuntu 8.10 was released but, just in case, you can remove that package (always assuming that no one using that computer needs to use a braille device).