iOS Artwork Extractor extracts all the artwork and emoji symbols contained in iOS 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 a folder on your desktop.
See instructions on the wiki to extract more artwork.
iOS Artwork Extractor can also generate glossy buttons png files if you are using a simulator running iOS 5.0 or older. Thanks to @schwa for the original code. Three states (normal/highlighted/disabled) glossy buttons png files are saved into the same directory as iOS artwork png files under the UIGlassButton subdirectory.
iOS Artwork Extractor also knows how to extract artwork from apps you downloaded from the App Store. It goes without saying that this is for educational purpose only and that you should not steal others' atwork.