I noticed a second button blank on the '06 dash photos. I just found what it's for. It looks like it offers 3 height levels for the HID. I wonder if it's standard on US or just for Japan. [Broken External Image]:http://toyota.jp/prius/safety/active/image/03-p06.jpg
that'd never make it past the NTSB, what have people adjusting their own headlights. Nope that'll only be in Japan.
I think that feature was an option on some Siennas, usually if you have the tow package as your "rear" gets pushed down.
The Prius with HID headlights has a levelling sensor that knows whether your rear gets pushed down by a big load in the hatch. It automatically adjusts the level of the headlights to match. So moving back to a manual switch would be backwards. In Europe I've heard that headlight levelling switches are common; the idea is to have some levels that are a compromise between "High" and "Low". This would never fly in litigious America because some idiot would blind oncoming traffic and sue the carmaker when they crash. I'm confident this won't be on the American version.
In most of the world, thanks to UNECE Regulations, headlight levelling control is compulsory. It can be either automatic or manual, but cars with intense lights (such as HIDs) must have it automatic. With a manual control, the default aim (0) is appropriate for an unladen car with a driver. You need to turn it up when you've got rear passengers and/or heavy stuff in the back. The control does not allow you to aim higher than that default. The Prius has manual control thumbwheel in steps from 0 to 5. HIDs are not offered in the rest of the world because the car doesn't have headlamp washers - these are compulsory for lights that bright according to UNECE. Presumably in the US there's no requirement to have any sort of headlamp levelling at all? But then you don't usually have proper cut-off dipped beams either, so it's less critical.