Photo Booth records video mirrored. There’s an option to flip photos, but I haven’t found one for video.
Recording video through iMovie is not mirrored. When using the iSight, however, iMovie always gets the audio from the built-in mic.
Quicktime Player can also record video (although it requires Quicktime Pro), and it will honor the system settings for audio input. This way I could record unmirrored video with the iSight while capturing the audio directly from my Digitech RP350.
(I had to search, though, to find that Quicktime Player was saving the movies to my Desktop.)
Next experiment will be recording video, guitar via the RP350, and vocals via a USB mic, which will require creating an aggregate audio device with the Audio MIDI Setup app.