Check out OpenUDID:
OpenUDID is a drop-in replacement for the deprecated uniqueIdentifier property of the UIDevice class on iOS (a.k.a. UDID) and otherwise is an industry-friendly equivalent for iOS and Android, and most recently Windows C# and Silverlight (see links above).
Edit: Make sure since May 1st 2013 for iOS 6 to use the this Branch until the Main is updated:
