In my office, I have a dual monitor set-up that I find to be very useful.  When on the road, I carry my laptop PC and an iPad2.  Air Display ($9.99 The App Store) allows me to use my iPad as a dual monitor over a common wifi network or via an ad hoc network.

Simply install the iPad app and the companion PC/Mac software and you are ready to go.  The App allows touch screen control on the Pad when paired in dual display mode.  I have found the App to work well and recommend it for those who need a mobile dual monitor solution.