LV's structured laptop cases (Monogram Cupertino, Damier Ebene Sabana, and Damier Graphite Steeve) will fit up to 15" laptops. You'll be able to secure the laptop with straps. There is also the option of the 13" Laptop Sleeve, which comes in Monogram, Damier Ebene, and Damier Graphite. It is a zip-around style -- the laptop cannot be secured by straps, as the inside is an open compartment. You might also consider a messenger bag or tote in addition to the laptop sleeve.
I looked at the MacBook Air yesterday at Best Buy. Very nice! Did you get the larger size? The sides of the 13" laptop sleeve will hug a 13" laptop, but the computer will still move around a bit because it is so thin. The 13" laptop sleeve fits easily in any of the larger LV messenger bags (Abbesses, Reporter GM, Icare, Porte-Documents Voyage GM, Brooklyn GM, etc.)
I got the MacBook Air 13" and am also waiting for an appropriate case. It is so thin that I'm thinking perhaps a well padded sleeve (once they come out for a 13" Air) will be good enough and then just carry it in my Beaubourg