Football Outsiders has been using the "speed score" as a metric since 1999; here's an explanation relative to the 2008 NFL draft and here are the 2009 results. What follows regards the 2010 NFL scouting combine.
The difference between a 40-yard dash time of 4.37 seconds and 4.43 seconds is 60 milliseconds -- just about half the time it takes the human eye to blink. To most people, it's imperceptible.
So why should people care about the difference between the respective 40 times of projected first-round pick C.J. Spiller and midround prospect Ben Tate?
Because of a historically effective metric suggesting that Tate might end up the better pro back.