If you're an instructor curious about how to introduce blogs in your courses, you may want to read this post by Daniel Lende at the new Neuroanthropology. He describes his experiences getting students to broaden senior theses, community-based research and even exams.
I should write up the way I handle student blogs for my large course, Biology of Mind, which is starting again this semester. To my knowledge, it's one of the largest blog-based student projects. But I'll have to get a moment to spare first....