unmonitored serial coms, when using Ubuntu Linux, would lock up the usb serial TX and temporarily brick the Uno. A thread on this is at the arduino forum here:
http://www.arduino.cc/cgi-bin/yabb2/YaB … 86088093/0
Follow instructions at comment #34 to flash the new firmware and fix the bug:
http://www.arduino.cc/cgi-bin/yabb2/YaB … 8093/30#34
How to put Uno into DFU mode without soldering can be found here:
http://www.arduino.cc/cgi-bin/yabb2/YaB … 2838/10#10
I am a week into playing with Arduino and Uno and the instructions worked for me like a charm.