I have a similar issue on my one-year-old GS350 with 9,000 miles on the odometer. Dealership has ordered a new driver's side mirror. It is an intermittent problem. The mirror will work fine for a while and then it will occasionally get "stuck" and not tilt down when I shift into reverse. It will, however, when I put it back into drive, tilt up to compensate for the down-tilt it didn't perform when I put it into reverse leaving me looking at the sky...
That is the only problem I have had with the car this far (much better than my Corvette... :( )