Script or Terminal command to switch audio output?

I can switch the audio output by going to System -> Preferences -> Sound and changing the "Sound playback" device from 'Autodetect' to 'USB Audio'.

I want to be able to do this via terminal (and script).

asoundconf (whether run from the GTK GUI or terminal) doesn't seem to do anything. I suspect it changes the *default* sound output, but not the *active* one?

For instance if I switch from 'Headset' to 'ICH7' via asoundconf-gtk (the only two choices I have, except for 'PulseAudio') but then go to System->Preferences->Sound, the 'Sound Playback' still reads 'USB Audio', and if I press 'Test' the sound comes out over my headset, not my speakers.