Preview at WWDC likely in June, followed by September launch.
Apple Officially Drops 'Mac' Name from OS X Mountain Lion
Though it still used the 'Mac OS X' naming scheme in press releases, Apple called the new system 'OS X Lion' on both the main product webpage and the Mac App Store product page [Direct Link].
As The Verge points out, Apple has completed the transition to 'OS X' across both the Mountain Lion product pages, and the press release announcing the developer preview.
We confirmed the official name change with Apple, who told us that the preferred full name is "OS X Mountain Lion".The Macintosh (as Tim Cook prefers to call it) brand is still alive and well, though Apple seems to be focusing that term on hardware, instead of software.
Lion was the beginning of a unification of sorts between the Mac OS and iOS. It was, as Steve Jobs put it, what would happen if a MacBook Air and an iPad "hooked up". iOS is based on Mac OS X and, at a fundamental level, there are more similarities than differences between the two operating systems. Dropping 'Mac' completely from the name of the OS solidifies the subtle, but important, distinction between hardware and software.
OS X Mountain Lion is expected to be released later this year.