It is tempting to attribute shimmy to bicycle frame geometry or material. However, bicycles don't have a monopoly on shimmy or "speed wobble". I rode a motorcycle for 10 years and can tell you motorcycles do it, too. It happened to me in a turn at 75 mph once. The next time you see motorcycles racing on a track, look closely as they go into a tight turn. Look at the front tires as they brake hard and lean into the turn - you can see the wobble. On a motorcycle, you can accelerate out of it, but is this an option when descending on a bicycle?
On a bicycle, you can often stop the wobble by grasping the top tube tightly with your knees. My bike has a shimmy at about 25 mph when descending with no hands. If I touch the top tube with one leg it goes away. I was descending at 45+ mph 2 weeks ago (holding on with both hands!) and had a very slight shimmy, even though my knees were on the top tube. When I let off the brakes, it went away. When I re-applied the brakes, it came back. I was too chicken to go faster than that, so I endured the wobble.
Tom