Ten One Design, the makers of capacitive styluses such as the Pogo Sketch, have released a demo video showing pressure sensitive drawing on the iPad.
The demo uses a standard Pogo Sketch stylus and pulls additional data from the iPad's touch screen to simulate pressure differences by measuring area of contact between the stylus and the iPad itself. They also incorporated palm rejection algorithms from their Inklet product to prevent detection of extraneous touches from your palm.
Unfortunately, they utilize private frameworks to access the data. Typically, Apple does not allow App Store apps to take advantage of private frameworks as they may change over time. Ten One Design is hopeful that the public UIKit framework could be updated to allow access to the additional data. If that happens, they plan on releasing their code as a free software library so it can be included in any App Store app.
