UIKit Artwork Extractor extracts all the artwork and emoji symbols contained in UIKit into png files.
In order to extract Retina Display @2x high resolution images, choose the Hardware > Device > iPhone 4 menu in the iPhone Simulator.
The png files are saved into
~/Desktop/UIKit Artwork Extractor-version where version is the SDK version.
UIKit Artwork Extractor can also generate glossy buttons png files. Thanks to @schwa for the original code. Three states (normal/highlighted/disabled) glossy buttons png files are saved into the same directory as UIKit artwork png files.