void Swizzle(Class c, SEL orig, SEL new){
  Method origMethod = class_getInstanceMethod(c, orig);
  Method newMethod = class_getInstanceMethod(c, new);
  if(class_addMethod(c, orig, method_getImplementation(newMethod), method_getTypeEncoding(newMethod)))
    class_replaceMethod(c, new, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
  else
    method_exchangeImplementations(origMethod, newMethod);
}
via devinsheaven.com

How to Swizzle, the awesome way. Using a method like this allows similar functionality to Rails' alias_method_chain, allowing you to extend core system methods on-the-fly even though Apple doesn't tell you what those methods do. This is a powerful technique for extending functionality, and hooking in to tightly coupled classes like UINavigationController and UINavigationBar.