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 