Preview at WWDC likely in June, followed by September launch.
Mountain Lion OS X 10.8.1 Improves Battery Life Somewhat
Ars writer Chris Foresman, after extended testing, discovered that the battery life of his Retina MacBook Pro review unit dropped 38% from its previous 8-hours.The same findings were reported by many others in Apple's own support forums, and there were some reports that the development versions of OS X 10.8.1 may have addressed this issue.
MacObserver has conducted battery life tests on the final release build of OS X 10.8.1 to see if there had indeed been any improvement. Their testing process included using an Automator script that launched several apps and navigated to websites. The script would cycle this process continuously. The tests were run on a 2011 Macbook Pro. Here are their findings:
First, they were able to confirm the drop in battery life when going from OS X 10.7.4 to OS X 10.8. The drop they measured was 33% (2 hours and 10 minutes) just from upgrading OS X to 10.8. The "No WiFi Icon" was a "fix" of suggested by one of their readers, but showed no appreciable effect. They did find that OS X 10.8.1 (beta and release) builds did improve the battery life stats, adding as much as 52 minutes in their testing. The overall numbers, however, are still well under their battery life measurements in OS X 10.7.4 (Lion).
Apple is currently testing OS X 10.8.2, but there's been no word if that build helps further improve battery times back towards OS X Lion levels.