After striking out at the Electronics Flea Market Saturday, I stopped at Fry's for a fresh pair of clippers and soldering tips, and picked up a small case too-- a Serpac M4. I used some free protoboard Laen sent me (THANKS!) to build a simple Arduino-compatible circuit to fit inside it, with a single button and IR LED, powered by three LR44 (AG13, 357, etc.) button cell batteries; basically a mutetater circuit with one LED instead of four.
Following Martin Koch's code example, I got the circuit to trigger my 350D's shutter, making it functionally equivalent to a Canon RC-5 remote control. Using an ATmega168 is overkill though: most pins go unused, and it nearly drained the three LR44 batteries when left on all night.
I figured putting it to sleep would solve the power consumption problem, so I modified the ArduinoSleepCode example from the Arduino Playground to make the D2 button both wake it up from sleep and trigger the shutter. It doesn't need to be awake for any time at all really before going into "SLEEP_MODE_PWR_DOWN": the wake up function triggers a shutter release event; currently it sleeps after five seconds. This will be a key feature of the mutetater and a more complete intervalometer-- using a button or a DS1337 clock alarm to wake up when needed.