Nice! I've also been thinking about something like this for my photos. I have a ton of photos littered in a pics folder. Would love an API would I could query on EXIF data.
Try a semantic desktop app. If I'm not mistaken, Tracker[1] can index EXIF tags (as well as other metadata formats) and it exposes a SPARQL[2] endpoint that you can query.
I've also been thinking the same thing recently. I have iPhoto but it sucks for a) big collections and b) sharing. For example, locally you can browse a collection on another local machine but it sends full res photos across the wire so it's completely unusable.
Image support is planned actually (just noticed that is not in the wish list, will add it) but has some big prerequisites. With photos you have to be a little more careful regarding permissions, which means that users and roles should be supported first.
[1]: http://projects.gnome.org/tracker/
[2]: https://en.wikipedia.org/wiki/SPARQL