Nowadays we have face recognition (or at least face locating and focusing) built in even in the cheap digital cameras.Imagine google image search working with face recognition ... on indexing each picture it can locate the faces in an image and index them.Then for example you can upload a picture of yourself and locate all the images that you have taken part in.You can find yourself on photos that you are even not aware of.The next level will be real image recognition where you upload a picture of an object (some nice place like temle, hotel or something) and you find all the related photos.Of course this will recuire a lot of computer power but for example google toolbar can always provide a choise for the user to take a part in the so called \\"grid computing\\".When you are not using your CPU, google toolbar can send a request for a packet that will be computed on your machine and the result can be send back to the server.
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Facial recognition software doesn't work well now.
"too much variability"
http://en.wikipedia.org/wiki/Facial_recognition_system
take a look at: http://www.polarrose.com/
they are currently in private beta