Phil Gyford has some intriguing thoughts on how Google Maps could develop in the years to come.
Imagine in, say, 2059 looking up a location on Google Maps and being able to dial the view back fifty years to see what that building looked like in 2009. Zoom back and forth in time to see how the place changed as decades flip by. That will be amazing.
Update: Google Earth already does historical comparisons. (thx, garo)