updated 04:55 pm EDT, Wed April 18, 2012
Many known issues still remain
Apple is now seeding a third developer preview of OS X Mountain Lion. The new build is listed as 12A178q, as opposed to the previous one's 12A154q. The OS is still saddled with a number of known issues, including several related to FileVault. Canceling the preview installer's restart, for instance, may prevent people from entering a password until Disk Utility is used to unlock and repair a volume; some installations on top of OS X 10.7.2 may fail if the older OS has FileVault enabled. FileVault accounts created before Lion won't work at all at in the current Mountain Lion preview.
Other issues affect components like Mail, iTunes, Time Machine, Java, and QuickTime. Little is known so far about the additions Apple may have made in the new build, except that Apple may be putting in more effort to get the Mac version of Game Center running. The finished version of Mountain Lion is due to ship sometime this summer.
The second preview added things like location-based reminders and higher-resolution art. The latter has helped back rumors that Apple is working on Macs equipped with Retina displays, mirroring the advances the company has made with iOS handhelds.