The Genius I spoke to recently told me that they had stopped using MacBook Airs at the Genius Bar (in that particular store, anyhow)...switched to basic 13" MacBook Pros, because the Airs were being stolen, and the Pros are the only ones Apple makes with Kensingtons.
The referenced MBPs were discontinued in October, so, to the best of my knowledge, Apple no longer makes a Mac that can be chained to its spot...good opening for a 3rd party solution if there's not one out there already. It wouldn't be pretty, but there'd be a market for it.
