Preview at WWDC likely in June, followed by September launch.
Apple Seeds First Build of OS X 10.8.1 to Developers
The release has a build number of 12B13 and Apple reports no known issues, but asks developers to focus on a wide variety of areas for their testing. The delta update -- where only items that have changed are downloaded -- weighs in at 36.4MB.
According to 9to5Mac, developers have been asked to test Active Directory, Microsoft Exchange in Mail, PAC proxies in Safari, SMB, USB, and Wi-Fi and audio when connected to the Apple Thunderbolt display.
The OS X 10.8.1 seed is available for download via the OS X Developer portal at developer.apple.com.