So, you're giving a lecture, and your students are all focussed on their handhelds, twittering back and forth or to/from the world. Read this article and you'll begin to feel differently about it all. As the article points out, at least they're not falling asleep!
The article has a very positive spin about what it calls "the backchannel". It's about people at conferences, but the positive notes and advice are something teachers ought to read and think about!