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.
edit: Seriously, Why? And why the down votes?
The API is deprecated, but so far not removed, there's a lot of lead-time, and iCloud is going to exist for user identification.
Are there truly that many apps whose assumptions are so brittle and developers so absent that this change will break them?
(Unless OP meant business models assumed in apps that need to identify everyone, even if they don't want to. That's valid, if less interesting.)