As noted by TechCrunch, Apple has alerted developers in recent documentation that it is in the process of deprecating access to the uniqueidentifier alphanumeric string that is unique to each iOS device.
Apple recommends developers create a UDID specific to apps.
Obviously, UDIDs were a security threat as marketers and advertisers (and worse) could follow your usage patterns and gather data through different apps.
Apple likely will continue to use the UDID for its iAds, GameCenter, subscriptions and other services it offers across iOS devices, or so one industry CEO thinks:
“I guarantee Apple will not stop using UDID,” predicts one mobile industry CEO. 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.”