The side view of a 911 has the same contour as a ..... wing. When you approach the magic speed, the car gets "light". This causes the chassis height to increase (however slightly) which allows more air under the car making it even lighter. The most pronounced symptom is a lighter feel to the steering and more susceptibility to cross winds. With the right combination of factors at play, you can loose control. Remember the shape of the 935's? Not necessarily pretty, but effective at speed.
Sherwood Lee
http://members.rennlist.org/911pcars