According to Apple Insider, members of Apple Developer Connection have been asked to focus testing on AddressBook, Graphics, iCal, Mail and Printing.
The delta release is 580MB and contains one known issue that affects the ability to play content purchased through iTunes in QuickTime.
Meanwhile, Apple has also released OS X Mountain Lion Developer Preview 2, almost exactly one month after the first Developer Preview. DP2 now requires a 64-bit kernel, meaning that kernel extensions to drivers to support 64-bit operation will be required.
Only systems that support a 64-bit kernel will be able to run DP2. As well as 64-bit software support enhancements, DP2 brings a few more notable changes. The Notes feature within Mail becomes a standalone app and there is an entirely new Game Center.