Displaylink allows you to display parts of your screen on different monitors (including tiny ones)
http://www.displaylink.com/products.html
Here’s a blog post about different methods
http://regexp.bjoern.org/archives/000211.html
Here’s a link to Nick Zambetti’s app for the iPhone
http://labs.ideo.com/2009/01/20/liveview-an-iphone-app-for-on-screen-prototyping/