updated 04:42 pm EDT, Thu May 17, 2012
Known issues include problems with FileVault, Time Machine
Apple has released a fourth preview of Mountain Lion Server to developers. The code is also listed as build 12S219n, and follows just a day after a third update of the main Mountain Lion preview. A number of known issues are listed in the Server preview; among others, there are several problems each in the use of FileVault, Time Machine, and Game Center.
Server Developer Preview 4 is available through the Developer Center portal. Apple still hasn't narrowed down a release date for Mountain Lion past a "summer" timeframe, but will likely use this year's WWDC for an announcement. The event kicks off on June 11th, and Apple has promised to unveil "the future of iOS and OS X," presumably during its standard opening keynote.