I have an Uno R1 with the 8u2 interface chip, and an Uno R3 with the 16u2.
I was having problems connecting to the R1 over the serial port, and found that /dev/ttyACM0 wasn't created when it was plugged in.
After a lot of trouble-shooting (it was seen on the USB ehci bus, but nothing was created/loaded,) I plugged in the R3 instead, and it was immediately recognized.
Is this a known problem? Is there something I can do to fix it? I'm using Linux Mint 12, which is based on Ubuntu 11.10.
I think the R1 version will show up as /dev/ttyUSB0. The R3 and the Mega show as /dev/ttyACM0.
I only get ttyUSB0, which is my GPS receiver. It does not show up as tty, which can be easily verified in the logs.
I also don't quite understand why switching from 8u2 to 16u2 would change the device ID such that it changes the name of the device. The Duemilanove would show up as USB because the FTDI drivers say their name is ttyUSBxxx, but the AVRs (as programmed by Arduino) use different drivers that say ACM. At least, that's how I understand it.
I wonder if I can load the R3 USB code into the R1, and maybe it will be recognized? As long as it's smaller than 8 kB of flash, it "should" work...
I installed the combined DFU and serial firmware for the 8u2 from the Arduino 1 package onto the R1 Uno, and it became recognized on the serial port (so, all Unos are ACM0) However, I couldn't get real communication using "cat" or "dd" or even a small program I wrote myself. This worked fine on the R3 Uno.
Then I explicitly set the baud rate and bit format on the device in my program, and now the R1 and R3 work the same. I imagine the R3 respects the default settings, or matches the initial default settings, for stty, whereas R1 does not. (stty < /dev/ttyACM0 shows the same for R1 and R3 when they're connected, but R1 doesn't work until those modes are re-applied programatically)
Now, the main problem is I'd really like it to not reset when I open the serial device. That probably requires custom firmware, and I'm going to have to go spelunking for a while. Maybe it actually isn't worth it; a fresh reset now and then might clear out the cobwebs or something 
Do you have a matching /dev/cu* device for these serial ports. It used to be recommended to use those instead of the incoming TTY devices.
SurferTim:
I think the R1 version will show up as /dev/ttyUSB0. The R3 and the Mega show as /dev/ttyACM0.
The R1 Uno shows up as /dev/ttyACMx .Its earlier boards (Duemilanove, early Mega etc) with the FTDI interface that show as /dev/ttyUSBx .
My R1 works fine on Ubuntu 10.04 .
jwatte:
Now, the main problem is I'd really like it to not reset when I open the serial device. That probably requires custom firmware, and I'm going to have to go spelunking for a while. Maybe it actually isn't worth it; a fresh reset now and then might clear out the cobwebs or something
Disable the auto reset with a 1-10 uF capacitor across GND & Reset ? That what I do, it beats faffing about with software fudges.