Images are geolocated on Google Maps as locations are pinpointed. You can browse by location or zip code and see what images have been uploaded so far. Images are matched up as closely as possible in StreetView to show the actual location as it appeared.
There's also an iPhone app as well -- the HistoryPin website explains what it does:
"Explore the streets - holding your phone up to the street, the app uses your camera view to display nearby images. By selecting the image, it can be overlaid onto the modern view to create an historical comparison, which you can toggle or fade between."