After having purchased my macbook air (the summer 2012 model) I'm now going down that crazy path of looking at getting a cover for it.
I have a few preferences before I begin:
It must be stylish
It must offer protection against all but drops
It must not scratch the casing (Some cases have zips that come into contact with the casing) see the konomo review below for the studs issue.
Konomo Envelope Review - Selected as looking like being the best leather envelope. The sleeve looks like one end is left exposed so I prefer the envelope.
Timbuk2 Crater Sleve - Allowing you the peace of mind to know you are not cooking your macbook when you place it in here due to its porous nature; I don't think this is ever an issue with my air it doesn't get that hot.
Under construction come back for more research and info - finally a product review.