I just tried a couple uploads to a Mega on 12.04, and all worked fine. No /var/lock/ entry (except whoopsie). Maybe something about the Uno and 12.04?
Have you looked in /dev/ to see the file attributes for the arduino?
cd /dev
ls -l ttyACM*
Mine looks like this:
tim@tim-desktop:/dev$ ls -l ttyACM*
crw-rw---- 1 root dialout 166, 0 Sep 16 12:33 ttyACM0