Apple has long announced
uniqueIdentifier UDIDs would be deprecated. From 1st May 2013, Apple will reject apps that use the deprecated identifier.
So what you have do?
DoubleEncore has writtern a good article on the other ways to generate UDID. It is up to you and your use case.
For me, OpenUDID is the nearest replacement. The only difference is that it cannot persist the identifier if there is a system reset.