How about a sort of distance metric from user decisions to help filter out things like the idiot FaceBook video spam that's been going around?
I warn you: this is currently on an incredibly vague conceptual level.
Let's take a webapp as an example, with a number of widgets that have onClick handlers; and let's say that some lovely lovely person has worked out how to exploit some XSS hole in order to programatically simulate a series of user actions to - for example - spam all their friends.
This metric is then sent back with every HTTP request.
Thoughts? I know this is hopelessly naive, but it may be a starting point worth examining?
 I know this is ducking the problem slightly, but it's a thought experiment, dammit!
 Obvious examples: anything that causes a reload or relocation of the window; anything that changes the DOM; anything that performs an action on a user's behalf.
The far-flung eyes of Orion are closer than you are to living breath. On a winter's night steam from my lips can reach upwards and make them shiver. I have spent hoarfrost midnights singing to Jupiter rising, and he has moved across my sky to reach me. But breath cannot fight winter; warmth cannot fight cold; and a single momentary presence from which you turned away cannot fight a defined absence. Entropy is the last dance spoken, in the greatest and the least; and if you drift away from me in complex currents, your hair caught and fractal with the weed, I can do nothing, not even follow.
I would cut into living stems to find you, curled up like a pearl, in the seed-pod of a poppy, and bring you back. A ghost in the rings of an oak; a conversation I had with a distant autumn morning, 4 AM and sodium. I would have come to you or brought you home or filled your heart with grammatical dreams, had you let me. But you are Ophelia in the frozen river; your silences are your ice, and your flowers.