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Longbets is kind of the same idea, but seems restricted to famous people (tptacek or pg would qualify I think).

I was not able to find a website that exactly does this. I feel like this idea might have big potential, and can go viral easily.

edit: seems like everyone can create a bet on that site.

http://iwagerr.com is a twitter-based platform for random, friendly bets people make with one another all the time.

Disclaimer: This was my "weekend" project a year or so back. A revamp is in order!