I've got Ubuntu 10.04 and got the same issue. I plug out the board, plug it in while holding the reset button and restarting the IDE. Then I get the message:
RXTX Warning: Removing stale lock file. /var/lock/LCK..ttyACM0
And everything works fine until I open the Serial Monitor again. Sometimes I get this message:
RXTX fhs_lock() Error: creating lock file: /var/lock/LCK..ttyACM0: File exists
maybe this helps somebody to solve the problem...