What a coincidence. I just got a 600D and was going to post the very same question. My subject will be birds landing on a feeder. I am used to writing CHDK lua scripts so I am looking into writing an ML script that changes Tv, Av, ISO just before a shot. This means you could have ML running with shooting settings bright enough to let motion detection trigger the shutter. The script would then change the settings to suit the flash, take the shot and then restore the brighter settings. Will post here if I can get it to work. (unless some guru has a simpler solution