Assuming access to the UDID – and absolutely nothing else – is... dumb. What happens when users change devices, for example? I'm having a hard time imagining any large number of apps that are that reliant on this one thing. Even OpenFeint had provisions to handle non-UDID-based account authentication.
This allowed a simple way to keep user data without worrying about additional setup, username, login, etc.
If it is gone (and not merely deprecated) then developers need to get an update out now that can read the ID and cross reference it to other code.
Otherwise, all settings that might be stored by a user on a server will be lost if the developer relied solely on using this ID.