Really excellent presentation slides and code by @mpospese here introducing matrix math as applied in Quartz2D, CGAffineTransform, and CATransform3D and working up to flip and fold animations:
Matrix transformations can make your user interfaces come to life: translate, scale, and rotate. Each on its own is relatively simple and straightforward. Yet many developers are daunted when 2 or more operations need to be combined. What if you need to rotate or zoom about an off-center (or even off-screen) point? How do you combine multiple transformations into a single animation? How do you make advanced, polished 3D animations such as folding and flipping views? Learn everything you need to know to get started with complex matrix transformations in CoreGraphics and CoreAnimation.
And once you’ve got that read, check out the massively detailed followups he’s blogged:
Just the ticket for adding all that trendy foldy flipiness to your apps.
And while you’re there, read the rest of the blog too, several useful tips on graphic tweakiness for you!