My previous frame was a 2002 LeMond Tourmalet, which had 853 main tubes and 531 stays, along with a steel fork. As such, there was nothing much to dampen resonant vibrations that happen with any metal object (think of it like a tuning fork). On the LeMond, this would happen around 40 mph, and the frame (which was verified as being in excellent alignment by two different frame builders) would start to shimmy. I'd clamp the top tube between my legs to dampen it. The shimmy would never get to a dangerous level, but it was certainly not subtle. It would subside again once I topped 42 or 43 mph.

My guess is that the LeMond's wheelbase length and the all-steel makeup of the build had a lot to do with its behavior. My friend who has a 2002 LeMond Buenos Aires in the same frame size and a carbon fork does not have quite as extreme a shimmy at speed - something I've verified on his bike. So the carbon mix seems to have a damping effect on the resonant frequencies.

Just my speculation, but carbon is used in many applications for its flexible damping qualities (e.g. skis, where different lays of the fiber can have different effect on the ski's behavior).