Apple is apparently starting to remove access to the UDID (Unique Device IDentifier) in iOS5. In any event, the best you can now do for identification purposes is to use a UUID (Universally Unique IDentifier). This has to be on a per-app basis. That is, there is no way to identify the device any longer, but you can identify an app on a device. As long as the user doesn’t completely delete the app, then this identifier will persist between app launches, and at least let you identify the same user using a particular app on a device. Unfortunately, if the user completely deletes and then reinstalls the app then the ID will change, but this is the best anyone can do going forward.
