Preview at WWDC likely in June, followed by September launch.
Apple Seeds OS X 10.7.3 Build 11D36 to Developers
The delta version of the new build for updating from OS X 10.7.2 weighs in at 986 MB, while a combo version good for updating from any previous version of OS X Lion weighs in at 1.2 GB.
OS X 10.7.3 will be Apple's next free maintenance update for OS X Lion users. The current version, OS X 10.7.2, was released in mid-October and brought support for Apple's new iCloud services.