If you want to roll your own solution with OpenGL ES, I've written an introduction to simulating a page curl using a conical deformation algorithm. While it's probably not the same solution Apple uses, it looks pretty realistic and can handle the general case of viewing the animation from any angle, not just straight on.
Since it's pure OpenGL and doesn't rely on any private APIs, it's also portable to other platforms other than iPhone OS.
While the curling itself is trivial to implement, you'll need to write your own finger tracking code if you want an interactive animation, which is a bit more complicated.