Here’s an interesting iPad app that turns your iPad into a secondary monitor. Shape Services iDisplay connects your iPad to your Mac desktop and lets you wirelessly extend your screen onto your iPad’s display.
It will also work on the iPhone, but I’m not sure why you would want to use your iPhone for this. Right now it’s version 1.0 product, so I’m sure they have some bugs to work out, but the users who have problems are using iPhones. It’s a great idea.