I found that on Tire Rack:

Speed Rating

In Europe, where selected highways do not have speed limits and high speed driving is permitted, speed ratings were established to match the speed capability of tires with the top speed capability of the vehicles to which they are applied. Speed ratings are established in kilometers per hour and subsequently converted to miles per hour (which explains why speed ratings appear established at “unusual” mile per hour increments). Despite the tire manufacturer’s ability to manufacturer tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits.

Speed ratings are based on laboratory tests where the tire is pressed against a large diameter metal drum to reflect its appropriate load, and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire’s required speed has been met.

It is true to say that I am used to matching the speed rating of my tires with the top speed of the car I drive since I used to drive a lot on German autobahn (equivalent of interstates), with no speed limit and pretty fast cars.
This is why I don't feel comfortable putting S rated tires on my Vette eventhough I do not intend to maintain sustained high speeds in the US like I used to in Germany.

Also, I agree with Norsky when he says

...you can't put a price on the loss of a limb or a life when things go violently wrong.

|UPDATED|10/21/2003 8:53:47 PM|/UPDATED|