Jobst Brandt has this to say about shimmy: "Bicycle shimmy is the lateral oscillation of the head tube about the road contact point of the front wheel and depends largely on frame geometry and the elasticity of the top and down tubes. It is driven by gyroscopic forces of the front wheel, making it largely speed dependent. It cannot be fixed by adjustments because it is inherent to the geometry and elasticity of the bicycle frame. The longer the frame and the higher the saddle, the greater the tendency to shimmy, other things being equal. Weight distribution also has no effect on shimmy although where that weight contacts the frame does. Bicycle shimmy is unchanged when riding no-hands, whether leaning forward or backward." Complete article at http://www.sheldonbrown.com/brandt/shimmy.html
I watched a friend almost go down two days ago during a shimmy at 35 mph. He stopped it by grabbing the top tube with both knees. Like JDF he was a tall guy with a big bike (steel).