Just got the 13 inch air and looking for a hard shell. The STM grip and the tech21 are my top two choices, please throw out any other names. Speck I heard is Awful and I'd rather go with something not cheap
I will second the Moshi iGlaze, I've got the black version on my 13" MBA. Great material, great workmanship, great product.
It's a bit more expensive than other alternatives, but in this case I got what I paid for after trying another brand which was ok, but in less than a year the tabs which held the lower case on to the chassis has broken off.