Yoshiさん こんばんは、 I think I have some good ideas to make language exchange over the net very interesting and workable for anybody.
a) authentication First, use Orkut or Mixi to provide evidence that the person is a REAL person. I mean, someone who has a photo, full name, a descent description of who he/she is, and some or lots of frinds, is a "relative good proof" that he/she exist for real. Once a person is in "REAL MODE", commitment is far greater than when the same person is in "anonymous mode".
b) limits and milestones Explicit writing statements about the exchange and how to process are going to happens will prevent a lot of expectation/frustation problems. In the current Internet, everybody EXPECT a lot, but didin't EXPLICIT declare what their expectation is and did not ask for a CLEAR statement from the otherside. Explicitly declaring and ask that : "I like to have an aproximate total 20 hours of english kaiwa within the next 30 days about greetings" - can create the right expectation to both partners and provide motivation for a lot of candidates to offer cooperation (exchange). By declaring and ask NOTHING, we usually get what we want, it's like a project without a deadline. So, stablishing some limits and milestones will help the cooperation set a target and fullfil that.
c) tools Genzai, Skype and PodCast is two great exemple of tools that is simple and powerfull todo any kind of language exchange. And, with one Gmail account, people can exchange TONS of mp3s with each other, without a need of a website. Of course, any good POP account works too.
We can create podcast to teach the concept and volunteer to Skype as "facilitators" to help the first groups of people interested.