Idea for later - a 1:1 teaching meetup network
catonmat.net
catonmat.net
Even though it appears relatively simple on first glance, it's actually quite complex to execute nicely. Especially the incentives part is tricky. http://www.liveninja.com seems to know what they're doing though, you might want take a look at that.
A friend of mine in Germany used to meet up with an American studying abroad at his university for 2 hours a week. One hour they'd speak English, the other German.
I imagine it's not that uncommon.