Old Map App allows an iPhone user to explore the effects of time on geography and urban development. While it isn't in general release quite yet, you can register at the site and you will be notified when it is released or when beta testers are needed.
The application displays layers of geo-referenced historical maps projected onto a modern coordinate system, so that the same location can be compared over time. Layers can be faded, adjusted, and explored freely. If the user is located within the region of the historical map, the user's position will be mapped on the old maps to the position of the compass indicator.
Several maps of New York City & Region are included from the 17th to 19th centuries. All are high-resolution images from the Library of Congress. I am an old map nut and can not wait!