Preview at WWDC likely in June, followed by September launch.
Apple Ramping Up New MacBook Air Production in July [Update]
According to Digitimes, Apple's notebook supply chain will "run in full gear starting" in July in order to meet production of the new MacBook Air models. Apple is said to be expecting to take delivery of over 8 million* MacBook Airs in the 3rd quarter. According to Digitimes, this represents a doubling over the previous quarter.
Shipments of parts and components for MacBook lineups totaled an equivalent of 2.2-2.4 million MacBooks in June, and orders for July are likely to top 2.7-2.8 million units, said the sources, noting that the increase is in line with Apple's previous strategy to ramp up deliveries prior to the launch of new products.The sources also reaffirmed what has been expected -- that the new MacBook Air will feature OS X Lion and support for the Thunderbolt interface.
Note that this contradicts previous rumors that Apple had already ramped up MacBook Air production. The July ramp-up seems more likely if you are to believe that the MacBook Air will ship with OS X Lion. OS X Lion has not yet seen a final Golden Master build yet. In order to pre-install OS X Lion on MacBook Airs, Apple would presumably have to hit this milestone. Note that separately we have heard that Apple is tentatively targeting the 3rd week of July for the MacBook Air launch, but that this date could always change.
*Update: Digitimes' 8 million MacBook Air Q3 estimate is questionable since this represents twice as many Macs (all kinds) Apple shipped in Q2 2011, casting doubt on the specifics of this report.