Make sure to get something that includes a layer of hard protection with a layer of padding against the machine. I've seen 3 MBA's with broken screens due to pressure or impact on the outside of the case. Thin has its price.
I just got an older MBA to see if it will do what I need for work. I haven't bought a case or anything for it yet, and am considering making a hard sleeve from Lexan, wet suit material, and shock cord. Probably with the Lexan thermo moulded to the MBA then sandwiched between layers of the wet suit material.
Dave