UIView-FlipTransition is a project mainly written in Objective-C, it's free.
An Objective-C category on UIView to make Flip transitions between Views
UIView+FlipTransform is a category on UIView that create a flip transition between two UIViews on iOS.
To use the UIView+FlipTransform in an app, just drag the UIView+FlipTransform class files into your project and add the QuartzCore framework.
Refer to the Demo included in the project for now. I'll add a more detailed explanation in future.