What our listeners can't see is Doug actually has sitting in front of him the world's largest laptop which not only has like a 19 inch screen on it it, and also has a pullout secondary screen. So in reality he doesn't need this.
But when I travel with my small laptop it is nice to be able to set up my iPad as a secondary monitor.
I've gotten used to be able to drag portions of my work on to a second monitor so I can look at it while I'm working with the main content. I don't have the second monitor with me when I'm traveling and my iPad would be perfect for that. And there is an app that makes it possible.
It works pretty well too. It's called Airdisplay and it's not free but it's definitely worth the cost. There's a piece that runs on the iPad and there's a piece the runs on your laptop computer.
It runs both on Windows and on Mac computers. If you really want to use it as an iPhone display you can have your iPhone as a secondary display well though that is so small I can hardly imagine that it's worth it.
In any case check out Airdisplay on the App Store to see how you can use your iPad as a secondary display.