I was going for the Viper hardcase, but it is hard to find in the UK and expensive. I was also not impressed with anything I found in the Applestore. The bags, what few there were, seemed all fluff and no substance. And expensive.
Enter the Targus Slim Case to the rescue. Costing a mere £16.99 delivered from Expansys, this is a solid, hard slipcase which will protect the innards from knocks as well as drops. It adds about 0.5kg to the weight, so not for those that really must travel as light as possible. Larger versions exist in black/orange trim. I had to settle for khaki/orange, but it looks nicer in the flesh than I feared.
The nice thing about it is that it can open from the top, like a normal slipcase or you can open the zip around the sides so that it flips open and provides a protective desk for when you are having to place your easily scratchable MBA on dubious surfaces. A pair of straps inside secure your notebook whilst travelling.
I'm chuffed to bits with it.