Open Culture has a really interesting article by a Pitzer College professor who taught an entire course on YouTube. It's a really fascinating exploration about the potential losses and gains to be had from teaching on web 2.0. In short, the professor did NOT like the experience and felt that teaching in this manner cannot replace a live classroom experience.
I'm particularly interested in this sort of thing because at the NAIS conference this spring futurist Faith Popcorn predicted that a lot of what we do in the classroom will migrate online in the next 20 years. This Open Culture article points out more than a few bumps in the road. I don't think that this article necessarily puts any nails in the coffin of this sort of thing, but I do think that it gives proponents of web 2.0's use in the classroom (myself included) something to consider before rushing blindly into the new frontier.