Recommended reading: http://sheldonbrown.com/gloss_sa-o.html#shimmy

In addition, try "high speed bicycle instability" or "... vibration" and similar searches. Many causes have been suggested, and it is likely that there is truth in each, in some cases, some of the time. Some wheel/frame/rider combinations seem more prone than others to this problem, which I used to encounter w/ my first Capo, which had a double-butted 531 frame and a Pletscher rack over the rear wheel.

Nishiki reportedly changed from a butted to a straight-gauge seat tube on its 25" Competition frames because of customer complaints about shimmy. The heavier gauge would alter the frame's modulus of elasticity and move its resonance frequency.

Other culprits have been mistracking wheels, even a mis-dished rear wheel.