If you're going to get a sleeve, I highly recommend the Sleevecase from Water Field. They are probably the highest quality and most protective sleeve out there with their 6mm neoprene padding(unless you go for a hard sleeve case, like the booq viper).
I handle my 13" Air somewhat rough (tossing it on the table, shoving it in my backpack full of stuff, tossing the backpack) when it's in the sleeve and it's been great so far.
+1 for Waterfield....have use them on several laptops...always great sleeves..