Hiya, I've been using synchronous and asynchronous support on my Open University physics Course this year. Interesting that the ones who use these facilities are the ones who also make the effort to attend face to face sessions.
I've found using synchronous chat useful for exploring very specific concepts and problem solving where it's hard to extend over a longer discussion.
I'm doing a project over the next couple of years specifically about synchronous support, but looking at things that go beyond chat (shared whiteboards, documents etc). I'd also love to work towards a state where students get together to work on specific tasks, synchronously, or where there's an open chat/session at the same time each work - I've heard lots of anecdotal evidence of these kinds of sessions working well.
One issue - if a student leaves a course - are his/her postings lost?