The source code for NSObject is available in the Objective-C Runtime (latest version available at the time of this edit) http://opensource.apple.com/source/objc4/objc4-680/runtime/NSObject.mm as noted above. Although Cocoa (Touch) the Framework is not open source if you wanted to see that. Additionally there is another complete implementation of NSObject in the GNUStep Base Package https://github.com/gnustep/base/blob/master/Source/NSObject.m.
