Autodesk, the makers of AutoCAD and Revit, have a cool drawing / painting app for the Apple iPad. It's called
SketchBook and it looks really easy to use and powerful. I will say that Autodesk has a way to complicate the simplest of computer commands (nothing is ever simple with them) but it appears that this interface is very intuitive.
At this writing, I don't own and iPad... but this looks to be one of the first apps I'll purchase and review when I do get mine (here in a few months). If you have used this app, let me know what you think.