There are some standard formulas that have been used by the 'pros' for a long time as good starting points. These formulas use a few basic measurements of the body. Longfemur mentioned one of them. They are described in more detail here:

http://www.coloradocyclist.com/bikefit/

Many cycling books go into even more detail. "Zinn and the Art of Road Bike Maintenance" has a nice piece on bike fitting. (Authored by Leonard Zinn)