The move comes after over a year of warnings from Apple regarding third-party use of Unique Device Identifiers, which are leveraged to track users for advertising purposes.
Apple's notice to developers:
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
Starting May 1, new apps and app updates submitted to the App Store must be built for iOS devices with Retina display and iPhone apps must also support the 4-inch display on iPhone 5
Because every cellular-connected mobile device has a UDID, the identifier can be used as an incredibly useful tool for advertisers looking to get the most granular data regarding ad impressions. Mounting concern over privacy issues, including location tracking on iOS 4, ultimately prompted Apple to begin deprecating use of the identifiers.
In March of 2012, Apple started to tighten access to UDIDs, reportedly initiating App Store rejections for apps using the per-device information. The move came after a 2011 warning that UDID access would be cut off with the release of iOS 5.
As for iPhone 5 screen support, Apple is ensuring a consistent experience for all iOS device users.