Did you try deleting the lock file(s)?
First, remove all serial USB devices (if only the Arduino, unplug that).
Then, in a terminal, change to the directory for the lock files (/var/lock/), then type (without quotes) "sudo rm {filename}" where {filename} is the name of the lockfile. Remove all that relate to ttyUSB0.
Reboot the machine, then re-plug in the Arduino, and try starting the IDE (you may want to do this at a command line to see if the error messages recur).
If that works, then you need to exit the IDE, and try starting it again - if that fails, then something isn't able to remove those lock files. Check the user/group/permissions on those lock files that are created; see if you are in the group, if not, add yourself to the group, remove the lock files, and re-try the steps. If there is a process for keeping rxtx from creating the lock files, you might have to figure that out.
Good luck, and I hope this helps! I've never run into this kind of a problem with my Ubuntu setup - but you never know...
