On Tuesday, 1 May 2012, Apple issued a software update for OS X Mountain Lion Developer Preview 3. Apple released Developer Preview 3 to developers.
The new update brings the build number of Mountain Lion up to Mountain Lion Developer Preview 3 12A193i (from 12A178q).
If you already have Mountain Lion Developer Preview 3 installed, choose Software Update from the Apple menu to download the update from the Mac App Store. Download Xcode 4.4 Developer Preview 4 from the Mac Dev Center to continue building apps for Mountain Lion Developer Preview 3 Update.
9TO5 Mac noted a change. This is regarding the addition of a “Do Not Disturb” setting to Mountain Lion’s Notifications Center:
Besides the Notification Center, Mountain Lion is also borrowing a number of other features from iOS, such as Reminders, Messages, and Game Center. Apple first previewed Mountain Lion in February 2012 and promises deliver to customers in late summer 2012.
Source: Mac Rumors