In Steve Jobs' keynote announcement of the iPhone SDK 4 earlier this year, one of the slides showed that a "Half curl page transition" was part of the new SDK:
I've looked through the iOS API docs and I can't seem to find this transition. Does anyone know where it is?
It's not a transition, but a modalTransitionStyle. You can check the docs here: http://developer.apple.com/iphone/library/documentation/uikit/reference/UIViewController_Class/Reference/Reference.html#//apple_ref/occ/instp/UIViewController/modalTransitionStyle
If you create a new UIViewController, set its modalTransitionStyle to UIModalTransitionStylePartialCurl, and then presentModalViewController:animated: it, you'll get the desired "half curl page" effect.
Are you talking about iBook style page turning animations? There's a blog piece about it here, with links to 2 GitHub projects with it:
http://oleb.net/blog/2010/06/app-store-safe-page-curl-animations/