A point is not necessarily 1/72 inches. TeX, for example, uses 1/72.27 by default. See http://www.oberonplace.com/dtp/fonts/point.htm for details on the different definitions of points. Even once you've settled on a definition of point as a unit of length, there's still the problem of defining what a "12 pt font" means; does it refer to the cap height, the length of an em-dash, or something else?