Fitt's Law is an incredible piece of research and its complete effects have yet to be seen, even today some fifty years after this paper was first published. The Information Capacity of the Human Motor System in Controlling the Amplitude of Movement, as Paul M. Fitts 1954 paper is called, is an astonishing piece of research because even though the paper is completely dry and long-winded, the author conveys a sense of simplicity in the ultimate math equation.
As has been proven to date, this equation has passed with flying colors to still be not only relevant but right. The application of this law can be seen in everyday HCI areas as menu development, icon activity, control panels and anything that deals with human movement and making it more efficent and intutive. I just wish the paper wasn't such a dry read. Alas, we can't always have our cake and eat it, too.