Last year in December, I spent a week wrote an API that processes up to 5 mobile inputs and cross references them to see if they are the same. The idea was to use multiple input factors to determine if two devices were close to each other (Color R.I.P - has some pending pattents surround this). They most challenging parts were matching audio and photo. I had the demo up and running on my iPhone in a week though.
Here is a view of the web interface that I would use to demo/test the API: http://i.imgur.com/tQA35U8.png
I still can't figure out a product for the thing, but if you have an idea, hit me up :)