How can I use my iPad as a graphics tablet?
The answer to your question is Avatron's AirPad software. http://avatron.com/apps/air-display I've just downloaded the Windows version and so far it seems stable enough, though I'm sure the Mac version is even more solid, since the Windows version appears to be relatively new.

That having been said, it's pretty impressive and does exactly what you want to do EXCEPT of course, there's no touch sensitivity, and there will always be a bit of lag (heck, Graphic tablets have lag too, believe it, but just a lot less).

Check it out!