Air Display 1.1
the author of this app says:
Ever wish you had an extra display for your computer? There's an app for that! With Air Display, you can use your iPad as a wireless display for your Mac OS X computer. Developed by Avatron Software, home of the popular Air Sharing apps, Air Display lets you position your iPad next to your computer, drag windows onto it, and interact with those windows as you would on any other computer display.
Compatible with iPhone, iPod touch and iPad. Requires iOS 3.1 or later.