I finally gave in to Twitter about 2 weeks ago. It is fun, certainly, and I'm getting pointers to interesting resources that I might not otherwise have found. I have yet to be convinced about its value in the classroom, or outside of the classroom, but I'm keeping an open mind.
Here's a post from Steve Wheeler at the University of Plymouth, giving his 10 top uses of Twitter for Teaching. He also has some links to other people who have used Twitter in their teaching.