PlaceTagger is a really cool application that records geographic coordinates to add to pictures. There is a desktop companion that takes the recorded data from the iPhone and adds it to the appropriate image file. This is especially cool for iPhoto users because you can see where the pictures were taken right there. All you do is start PlaceTagger, set the camera's clock in sync with the iPhone, put the iPhone in your pocket (the screen goes off automatically to save batteries), and shoot away.
Very slick, and far cheaper than adding GPS to a camera, if it is even possible to do so.