Why?

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.)

Because developers have used it ID to store user information on servers, etc.

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.

But will it break lots of apps?

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.

Sometimes users have different things on different devices.

Hell the requirements for auto-renewing subscriptions pretty much FORCE you to track udids.

No, it won't. This is a deprecation. The API will still be there, and it will still work. Using the API will now have a compile-time warning that developers will see.

At some point — far off, in the future — the API might be removed. But, keep in mind: there are APIs that were deprecated in iOS 3 that Apple still keeps around for compatibility purposes.