I have a very large photo gallery with thousands of similar people, objects, locations, things. The majority of the people in the photos have their own user accounts and ava
If you want to do it real-time in PHP, there is a way. Check out the solution presented here: it was mentioned on News.YC a month or two ago.
Pittsburgh Pattern Recognition, as you mention, seems to have some pretty good technology, and they have an SDK available: http://www.pittpatt.com/products/ftr_sdk/sdk_users_guide.html. They also have a web demo, so if you wanted to be sneaky, you could probably hack something up that would just feed images into their demo. They'd probably stop you eventually, though - better to contact them directly.
Check out PicsMatch.com! I've been quite pleased with it, but even the simplest browsing tacks are quite slow. Well worth the low price.
Check out face.com. They opened up their API yesterday.
Note: Face.com is no longer offering their API. (thanks facebook...)