Apple announced it was deprecating the Unique Device Identifier back in August of 2011, and over 18 months later the company will formally start rejecting apps that continue to use it. In a brief post on Apple's Developer website, the company states that it won't accept apps that use UDIDs as of May 1st, and encourages developers to use two other options to provide similar functionality should they need it.
UDIDs, which identify a specific phone or piece of hardware, fell out of favor as Cupertino became more focused on privacy concerns. Some app developers had used UDIDs to track users, as the identifier is permanent and users were unable to block its transmission. Some apps were reportedly rejected for UDID use as far back as March of 2012. However, during that time the UDID was still just marked as deprecated — meaning that it eventually would be removed, but hadn't been taking out of commission just yet.
On the same date, Apple will also start requiring that new apps and app updates support both Retina display resolution and the larger, 4-inch display on the iPhone 5. This effectively forces developers to use the iOS 6 Software Development Kit — and means older devices like the iPhone 3G and iPod touch (2nd generation) won't be compatible with future updates.