I had a good look at the Macbook air yesterday first hand, and the underside can be unscrewed, so you can gain access to the battery. The guy in the apple store did say it would invalidate the warranty though
I must admit that the warranty thing doesn't bother me personally (especially after having taken a mac mini apart last month with a putty knife to add an extra gig of ram and put a faster drive in), but I can see that it would annoy people having paid so much money for a machine.