The apps will present "an ongoing stream of nontextual, para-narrative, and extra-narrative stuff," and even ask readers to interact and create their own stories in the universe with some "pretty cool tech." Interesting.
There's not a lot of information out right now about what the project is exactly, but there is a Facebook page with a few more details, and a skeleton page where you can sign up for more information. I guess they're taking the wraps off of the project in a few days on May 25th, so presumably then you'll be able to download the app (which will also be available on Android and the Kindle), and see what it's all about.
I'm a big fan of Stephenson (as anyone interested in computers and how they work probably should be), so I'm definitely intrigued about what he and his colleagues will do with Apple's platform. Stay tuned.