Here is a good article on the history and current state of satellite imagery.
Satellite image of Area 51, captured by one of GeoEye's satellites.
Did you know...
There is one residence on the planet famously obscured by Google Earth: the Naval Observatory, official home of the vice president of the United States. Yet even Dick Cheney can’t escape the long arm of the Internet—a clear image of the Naval Observatory is available on Yahoo.