I'd like to extract the default UIBarButtonItem icons from the iPhone SDK. I imagine they're probably stored in the iPhoneSimulator platform as alpha-channel-only PNGs, but I've yet to find it.
The one I'm looking for is UIBarButtonSystemItemReply. (For those suspicious that there's even a valid use case for this, I'm looking to use this on a table row header where the user can post replies, row-wise)
I don't know how to do this, however I was curious about the same thing a few months ago. You may be able to initialize this UIBarButtonItem and extract the image from it by looping through all of the elements in its UIView and dumping the NSImages. I'm not sure exactly how to go about doing that, but I remember Erica Sadun wrote an article about it regarding using a full-screen camera image. I'm not allowed to add a link to it, so just Google for "erica sadun full screen camera".
The Other.artwork file is in /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator3.0.sdk/System/Library/Frameworks/UIKit.framework/ (you need the SDK).
Use the "iPhoneShop-1.3.jar" program -- available currently here to extract all of the images into a directory.
java -jar iPhoneShop-1.3.jar ARTWORK Other.artwork EXPORT Other