The way I look at it, if you have to turn the dimmer knob every time you use the headlights in the day, it should actually be easier to hit an override button to cancel the dimmer alltogether, wouldn't it? Then when you un-cancel the dimmer, your dimmer settings are left unchanged.
If you're really slick, you could wire a momentary switch to a latching relay. That way when you turn the lights on, the dimmer is active. Hitting the cancel button cancels the dimmer effect, hitting it again toggles it back. But the next time you turn on the lights, regardless of the state of the dimmer, it's is back to normal. That would make the operation identical to the SVX's dimmer cancel button, which I've found seems to work pretty good.
If you want to really go the stealth route... go to radio shack and pick up a light sensor and some relays. I'd have to think about it a bit, but I think you should be able to work out an auto cancel setup w/o having to go to a digital circuit. I just don't know what happens if you're driving around at night... would the street lights constantly toggle your dimmer on and off?
