It appears the new MacBook Air’s got a problem with transmitting via long VGA cables. A few days ago Christian Heilmann wrote:

I set up on stage, opened the shiny new expensive laptop, connected my VGA cable and saw some blue bars – that’s it. The new Macbook Air does not connect to projectors via VGA with long cables. We verified that with mine, Matt May’s and some other speaker’s Macbook Air in two rooms, with different cables, different connectors, projectors and all the settings we can think of. It seems the great shiny new Thunderbolt connection is good for file transfer but underpowered for projection.¹

Lea Verou reported the same problem on twitter, but then found and reported the following a solution:

@LeaVerou: Apparently, the MacBook Air VGA problem is solved if you boot it with the adaptor connected! CC @codepo8 @robhawkes²

A lot of technical problems at my workplace is related to projectors and presentations. To accommodate old pc laptops a few of our lecture halls still has VGA as their main, or only, means of communication between computer and projector. Given the success of the MacBook Air it’s likely that this problem will arise a lot in the coming year or two. Therefore I’m writing it down here, for me to remember and for others to find.

Rebooting the computer with whatever cable links you to the projector already hooked up is usually a good thing to try whenever you get a problem like this, regardless of whether you’re using VGA or not and a new MacBook Air or not.³