If it makes you feel any better, I have an extremely similar problem, maybe the exact same problem. Under Ubuntu 9, the arduino IDE properly detects the /dev/ttyACM0 port created when I plug in the arduino, but processing apps don't see /dev/ttyACM0 and therefore can't ever talk to the arduino. I downloaded the latest versions of processing and arduino yesterday to try them out under Ubuntu; so far, no-go for processing.
The only difference between your problem and mine is that my processing apps DO see my USB mouse port, /dev/ttyS0. Of course, you can't connect to that port (it's in use) and even if you could, it's not where the arduino is anyway (as verified in the arduino IDE and by dmesg upon connection of the arduino.)
SO, it would be really cool if someone could provide us some leads about where to look to solve this problem. 