Such a thing has existed since iOS 2.0 (CFUUIDCreate [1]), and Apple's updated docs on UDID specifically recommend using CFUUIDCreate. The problem is that an application-specific UUID doesn't address all current use cases for UDID. Specifically, ad networks that support Cost-Per Install (CPI) need to use an identifier that crosses the application boundary.
[1] https://developer.apple.com/library/mac/#documentation/CoreF...