As I understand it, pinners can post a single photo or a series, add to the number of existing photos at one location or create an entirely new series. It's particularly handy if the target location is on a streetview of Google Maps, but it isn't absolutely necessary.
When the story aired, there were about 55,000 photos pinned to the Historypin world map. Within a day that grew to more than 60,000. Genealogists could multiply that number exponentially. Have fun with Historypin - a brand new way to share family pictures.