Apple has pushed the third preview of OS X Mountain Lion to developers. A number of known issues are still present in the preview, including booting issues with FileVault enabled and installing Mountain Lion on a system running OS 10.7.2 or earlier with FireVault turned on could fail.
Other known issues include involve sandboxing apps, iTunes, Mail, a delay in Back to My Mac, DVD Player, Time Machine, QuickTime and a number of problems with Game Center. Full release notes are available in Apple's Developer Center.
The second developer's preview was released on March 16.