Here is a PDF document based on the research by Shumin Zhai, an IBM Research Staff Member, which extends the law developed by Fitts in 1954.
Fitts Law is a robust model of human behavior which enables the prediction of human movement and human motion based on rapid, aimed movement other than drawing or writing.
In Human Computer Interaction (HCI) Fitt’s law is a useful guideline in interface design.
There is also a great Interface Design blog entry in 37Signals titled, Better PDA Interaction, posted by Ryan Singer which looks at an OSX dock technique used in new PDAs.
Here are some further reading on Fitts Law:
1. A summary and examples of Fitts LawKDE User Interface Guidelines.
2. Usability First Glossary of Fitts law.
3. Ask Togg Quiz on Fitts law.