I have a plan to make a game using OpenGL for 3D world view, and CALayer(or UIView) for HUD UI. It's easy to imagine performance degrade from mixing them, but the document which mention this impact disappeared: http://developer.apple.com/iphone/library/technotes/tn2008/tn2230.html I cannot find the document on current version of SDK reference.
And I got this document: http://gamesfromwithin.com/gdc-2010-the-best-of-both-worlds-using-uikit-with-opengl
If you experienced about this, please let me know about performance impact on current SDK.