When working in UIKit developers are warned now to use images that are larger than 1024x1024 because that is the limit on the size if a texture in openGLES. I wouldn't be surprised to see this being doubled for the retina and Apple using that as the hard limit.

( You should avoid creating UIImage objects that are greater than 1024 x 1024 in size. Besides the large amount of memory such an image would consume, you may run into problems when using the image as a texture in OpenGL ES or when drawing the image to a view or layer - http://developer.apple.com/library/ios/#documentation/uikit/...)