While searching an address on maps.google.com, I noticed a link on the top-right of the web page saying "Satellite". Clicking on it made the map actually come live with satellite images of that exact area. Depending on which part of North America* you are looking, you can see images with a resolution to upto 1m/pixel! One can easily distinguish houses in densely populated areas like Detroit. I was also able to find the Twin Towers site in less than a minute.
This is a part of Google's aquisition of Keyhole, the satellite imaging company. The take over also included an imaging satellite, which has been rechristined as "gSat". Another big thing from Google(TM).
*-Unfortunately, Google still thinks that Earth has only one landmass, i.e. North America. The easiest way for them to learn is a Google Search.