As noted by TechCrunch that Apple is now making a huge change in iOS 5, phasing out the ablility for developers to access devices's UDID (Unique Device Identifier). On the other hand Apple asking developers to creat UDID specific to their apps to be able to link between installations and specific users.
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.Why Apple do this update ?
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.[via]