Launching with iOS 8.1.
Apple Phasing Out Developer Access to UDIDs in iOS 5
This is a big deal, especially for any mobile ad networks, game networks or any app which relies on the UDID to identify users. Many apps and mobile ad networks, for instance, uses the UDID or a hashed version to keep track of who their users are and what actions they have taken. App publishers are now supposed to crete their own unique identifiers to keep track of users going forward, which means they may have to throw all of their historical user data out the window and start from scratch.Apple and a number of app developers have been sued over their handling of UDIDs. While UDIDs can't directly be linked with a specific users, information tied to a device can be passed along to advertisers to help them in targeting their advertisements, with some privacy advocates objecting to the practice.
With the UDID, ad networks can track what apps are being used on a given device, enabling them to piece together a valuable picture of activity conducted on a specific device. Apple's move seems to specifically address that concern, breaking down identifiers to the app level to limit the ability to put together such a complete picture.