There have been some rumors that Apple will release the new MacBook Air this month. I doubted these reports as the release would be very close to the OS X Lion release. Releasing new hardware just weeks ahead of a new OS update just does not make much sense. Shipping new 2011 MacBook Air with the shiny new OS X Lion just makes a lot of sense.
Now AppleInsider has published a report that Apple is just doing that according to their sources. Apple is holding the MacBook Air, which would be apparently ready to ship early until they can ship them with OS X Lion pre-installed.
You can get OS X Lion update for free when you purchase a new Mac after June 6. So there would be no financial disadvantage for customers to still buy a OS X Snow Leopard Mac, but it is more convenient and Apple is all about ease of use.