Fast Company's Drew Neisser has a good column on public speaking in the social media age. I thought two of his points are worth emphasizing here, though I recommend reading the whole column.
First, he argues: "Don’t Panic if They Aren’t Looking at You." They might be on a portable device, such as an iPad or iPhone. However, it doesn't mean they are reading email necessarily. They might be taking notes, or tweeting about your presentation in real-time. Both of those behaviors could be very productive and useful for you. If they tweet your message, you could be reaching a much broader audience. Telling them to put away their devices is NOT the answer, according to Neisser.
Second, he explains: "If You Don’t Speak Twitterese, It’s Time to Learn It." In other words, you need to share your Twitter handle up-front. In that way, people can share their thoughts about your presentation, and you will be able to see those statements. Moreover, you need to provide some concise phrases and sound bites that will be amenable to tweeting. Think of capturing your message in a few phrases of 140 characters or less.