It's been done before, and to be honest it has been done better, but still it's nice to see the National Archives allowing us to search historical images on a map. They have started with a collection of 14,000 photographs from the 1920s-1940s, and promise more.
It's an interesting start, and fun to browse. I've voted for "quite useful" on their survey page, which is a bit less enthusiastic than most others. The specific feedback shows that some people seem to be having browser problems, but it works fine for me. My reservations are that it would be nice to see more precise locations, a bit more information the individual pictures, and a way to mix text and location searches. Perhaps all that will come (via crowdsourcing?), and meanwhile it is definitely worth taking a look. The link is here.