I have spent the last two days fighting a problem which has been experienced by many others. The Tools/SerialPort item is grayed out, and any attempt to send a compiled program to the Uno fails. I finally got it to work in the following manner:
A working blinker program was installed on the Uno from a Windows 7 machine.
The Uno was carried over to my Ubuntu laptop, and the USB cable was left unconnected.
A 9V battery was connected to the Uno via the 2.1 mm power jack. The blinker was running.
I then connected the USB cable to the Ubuntu, and the Serial Port showed dev/ttyACM0. Everything is working normally now.