Library of Congress Photos on Flickr should be an interesting experiment in how well social tagging can sort out content. It meets the criteria of having a lot of content that is currently very hard to find, and comprehensive recall is not essential (if some photos remain unfindable despite the social tagging, it’s a shame but not a disaster). The Library presumably has decided it would rather have any tagging than none and is reluctant to spend money. It also has a high profile and most likely the good will of the experienced taggers of the Flickr community. I would think it would also provide the librarians with a good starting point for organising their image metadata if they decide they want or need to do some more formal sorting out further on down the line. Definitely one to watch.