I am having the same issues as skylerl. I was able to use the board previously, but now I cannot; I am not sure what changed. I am running on a Debian based system (Mepis 12), 64 bit.
When I plug the board in (Uno), I can see ttyACM0 in /dev, and dmesg reports:
[ 108.286268] usb 7-1: Manufacturer: Arduino (www.arduino.cc
[ 108.286272] usb 7-1: SerialNumber: A4132373935351817271
[ 108.361005] cdc_acm 7-1:1.0: ttyACM0: USB ACM device
[ 108.364265] usbcore: registered new interface driver cdc_acm
[ 108.364271] cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters
and when the board is not plugged in, ttyACM0 is not in /dev. There is no file in /dev/lock/. I have put /dev/ttyACM0 in the preferences.txt file in .arduino/ and the serial.debug_rate=9600.
Running as root makes no difference.
I will note that I was experimenting with serial communications with an Android device using Java, and the instructions lead me to install librxtx-java, but I have since removed that package. Also, when I run the arduino IDE from the command line I get:
Experimental: JNI_OnLoad called.
Native lib Version = RXTX-2.1-7
Java lib Version = RXTX-2.1-7
so I don't think there is a conflict there (I read about that somewhere in my search for a solution). The board is fine, because I was able to upload a sketch to it using a Windows computer at work. I have searched the Internet for hours and tried everything except for reflashing (is that the correct term, so much of this information is now a blur and possible solutions are mixing in my mind) the arduino board.
I don't know what else to use for search terms, and would really appreciate any advice the experts out there may have.