Why is Apple preventing application developers from accessing the UDID? Two possible security reasons for this are to protect user privacy and to protect the secrecy of the UDID.
User privacy: Though the UDID identifies the device and not the user, many applications have access to both the device UDID and the user's identity. Such applications could be used to create a mapping of users to UDID which could then be used to identify users using applications that don't have access to the User's ID. For example, let's say a user has both a Facebook app and a porn app on their iOS device. Assuming the porn app doesn't require the user to enter any personal identifier, the user can assume that the distributors of the porn app don't know his identity. But since the Facebook app has access to both the UDID and the user's Facebook account, it is possible (for Facebook) to generate a mapping of UDIDs to Facebook accounts and this could subsequently be used by the porn app distributor to identify users of the porn app.
UDID secrecy: There is reason to believe that some Apple applications use the UDID as a secret value. Apple cannot rely on other applications with access to the UDID to keep this value a secret.
A conspiracy theorist might think that in fact the reason Apple is doing this is to keep the UDID to itself. As TechCrunch notes:
If Apple does continue to use UDID for itself but denies it to developers that would be an “extremely lopsided change.” It would give Game Center and iAds yet one more advantage over competing third-party services.In this case I'm voting with the conspiracists. If Apple wanted to protect the secrecy of the UDID or to protect user privacy they could have easily replaced the current UDID API with a new API that would still give the application developers a reliable unique identifier of the device but wouldn't compromise the secrecy of the UDID nor the privacy of iOS users. Specifically, Apple could have done this by replacing the UDID API with an API that returns a one-way function hash value of the UDID which is keyed using an application specific identifier (e.g. the App ID).
I'm sure the security engineers at Apple are smart enough to have thought of this. The fact that they didn't do so but simply removed the UDID API tells me that their goal was not to enhance security but to gain exclusivity on the device identity. Having such exclusive ownership of devices' identity gives Apple a great advantage in the development of identity-dependent applications including DRM.
Sometimes security is just an excuse for increasing control.