Stefan Wehrmeyer has developed a Google Maps application called Mapnificent which provides several intereting functions that combine maps with time. The application shows the areas you can reach with public transport in a user-defined time, and tis also displays the overlapping areas that two differnt persons can both reach within a given time.
Check the video clip in which the developer expains how this application works.
Mapnificent from Stefan Wehrmeyer on Vimeo.