Good ol' http://oo.apple.com http://oo.apple.com. It's a pretty decent opt-out method, but it only works because something hardcoded in the iOS version of Safa

Good ol' http://oo.apple.com http://oo.apple.com. It's a pretty decent opt-out method, but it only works because something hardcoded in the iOS version of Safari sends the UDID as a X-Header in the HTTP request headers -- specifically to the oo.apple.com domain (and a handful of others, all owned by Apple.) If any other company wants to offer a systemwide opt-out for its iOS software, it's a lot more difficult.