Considering we don't even know precisely when ML will be released, except that it will be sometime this month, it's hard to say. Last year they released Lion and the new MacBook Airs the day after their quarterly earnings release, which is scheduled for July 24th this year. Maybe they will release Mountain Lion the next day. In any case, expect the Airs (and all other Macs) to ship with Mountain Lion preinstalled soon after that.