A subtle change has been noticed in the iOS 5 beta. Apparently, Apple is changing the way that developers can access an iDevice's unique identifier number. For those unfamiliar with the concept, a 'UDID' allows developers to identify app users on a specific, device by device basis.
In iOS 5, Apple is no longer allowing developers access to an iDevice's UDID, but instead allowing developers to create app-specific UDIDs. This basically means that mobile ad and game networks won't have access to your device's specific number for tracking and inventory, which is the means that these systems have always worked off of in the past.