Rule #4 here is the most challenging (9 tips for running more productive meetings 43 Folders) at Microsoft:

4. No electronic grazing. Period. - Laptops closed. Phones off. Blackberries left back in the cube. You’re either at the meeting or you’re not at the meeting, and few things are more distracting or disruptive than the guy who has to check his damned email every five minutes.

I feel bad for some folks up in-front of an important audience and ready to do a big presentation or demo and all of the audience is staring at their screens.

This usually leads to the "huh-wha?" moment when something important enough is said to get a person's attention and now they are busy coming back up to speed on everything that's been said.

You can't do email and pay attention at the same time. I'd be damn happy for it to start being a requirement that the laptops are closed when the meeting begins, or at least folded downwards for easy access should you need to look up something relevant to what the room is discussing.