Apple’s iOS device UDIDs are commonly used for advertising and application user tracking in iOS. Earlier this year, Apple announced that iOS 6 will no longer support UDIDs. Instead, UDIDs will apparently be replaced, for advertisers at least, with a new feature called the Advertising Identifier. This identifier, unlike the UDID, does not tie personal device information to the ID. It is also non-permanent. Apple explains in the iOS 6 settings:
iOS 6 introduces the Advertising Identifier, a non-permanent, non-personal, device identifier, that advertising networks will use to give you more control over advertisers’ ability to use tracking methods. If you choose to limit ad tracking, advertising networks using the Advertising Identifier may no longer gather information to serve you targeted ads. In the future all advertising networks will be required to use the Advertising Identifier. However, until advertising networks transition to using the Advertising Identifier you may still receive targeted ads from other networks.
Along with this new identifier, Apple allows users to limit ad tracking in iOS 6. Apple says that some advertising systems used by App Store applications may still track information until they are updated to support the new ID system.