App support is still in progress. There is currently a demo app on the iTunes store (http://itunes.apple.com/us/app/pressurepaint/id515016023 ) and I'm in contact with a handful of developers that would like to incorporate pressure sensitivity into their notebook and sketching apps. If you have a favorite app to draw in, be sure to contact the developer (there's usually a support link in the App Store) and ask if they would consider adding support.
I expect that at least a few apps will be available when PressurePen units make their way into supporters' hands. Watch the updates page for announcements.
If you're a developer yourself, send me a message, and I'll provide you the latest source for the demo app to take a look at, and build into your own project.