I had shimmy under braking. I changed control arm bushings and subframe bushings with only a slight improvement. The rotor runout was found to be .005 inches while mounted on the car. Mounted on the lathe it was only .001 inches or less before turning. This means my hubs are not true. Bentley says for a standard type rotor you should have less than .002 inches runout. I also found a bad tie rod end which hasn't been fixed yet.
It worked! (10/97) The shimmy has been bannished even with untrue hubs and a bad tie rod end. (as of 3/00 it is still true.)
I think that it is an uneven drag around the rotor that causes the shimmy, rather than it being out of true. Let's say that the car sits in the rain for a few weeks and the bottom halves of the rotors get more rust pitts than the top half. You would now get shimmy because the rotor has more "grab" around part of it's circumference. That's a theory anyway.
Some friends of mine are trying to improve quattro brakes. Braking Improvement Research Association