Limner is an iOS app that turns an iPad into a pressure-sensitive graphics tablet for a Mac. The computer shares its screen via VNC with a client on the iPad, which sends touches back to the computer as a virtual mouse. Limner works with many Bluetooth styli. It is available on the App Store.
I single-handedly designed, built, and released both the iPad app and its companion server app that runs on Mac OS X.
Visit Limner's website, with much more detailed information