Here is a new (to me) use of Google Earth: Gary Hodges has made a network link that maps the migration patterns of early Pennsylvanian settlers as they head westwards from the port circa 1750-1850. He locates Indian villages and forts as well. More info on his blog.
Which reminds me of an early item on the wishlist for a future feature in Google Earth: The ability to play a KML file as a series of timed events, so that, for example, we can see historical battles played out, see the spread of the avian flu virus over time, or even just watch a hurricane as a series of time-lapse images. KML files certainly already have the ability to time-encode individual objects (though the feature is undocumented), so they could be shown only at specified time intervals if there were a play button to be pressed. Historical geographic information already encoded in KML files, such as many of the WWII overlays uploaded to Google Earth Hacks, would certainly benefit from the added metadata.
And, thinking a bit out of the box, imagine Wikipedia linking from its articles on historic battles to network links of those battles, accurately positioned in both space and time. If there were sufficient amounts of such historical KML files, properly time-coded, we’d be able to engage in grand tours of history, incorporating antique map overlays, battle sequences, migration routes and national borders, all of them changing dynamically as we watch the years tick by. Basically, we’d be smearing out all Google Earth has to offer into the fourth dimension. Not an entirely new thought, perhaps, but worth savouring every so often:-)