Apple warns developers it will stop accepting apps that access UDIDs on May 1

It’s been hinted at and implied in the past, but Apple(s AAPL) is now serious about apps using universal devices identifiers (UDIDs): starting May 1, such apps will be rejected from the App Store, the company told its third-party developers on Thursday.
On its developer site Apple writes:

Starting May 1, the App Store will no longer accept new apps or app updates that access UDIDs. Please update your apps and servers to associate users with the Vendor or Advertising identifiers introduced in iOS 6.

UDIDs, which were intended to be anonymous, have been used by publishers, developers and advertisers to track their app’s usage and more accurately target advertisements almost since the App Store opened in 2008. But there are huge privacy implications with that practice: with just a bit more identifying data, a device’s UDID can be traced to a specific owner.
Apple first mentioned it would start rejecting apps that used UDIDs, due to privacy concerns, in late 2011. Then almost exactly a year ago, there were signs the company’s app review team had begun enforcing that rule. In September 2012, Apple introduced a replacement system for advertisers to use — the Advertising Identifier, an anonymized number that users can choose to reset, or opt out of altogether.
By May 1, the Advertising Identifier will have been available for eight months; plenty of time for those who want to understand how their apps are being used to switch over to the new system.
Besides UDID use, 9to5Mac notes that App Store reviewers will also be looking for apps that aren’t optimized for Retina displays and will reject them, along with iPhone apps that are not optimized for the 4-inch screen of the iPhone 5.