My MBA is about a year old and has NO scratches on the case. It's only been out of the house and on the road a few times though. My iPad (also a year old) does have some minor scratches on the bottom of the case. The metal appears to be the same or similar to the metal on my MBA.
The one thing that can definitely mar the surface is if you drop something sharp on the case. That happened to me on my previous MBP, resulting in a very annoying small dent. However, judging by what other MBPs were selling for a year ago, I reduced my asking price $100 and didn't have any trouble selling the computer with the small dent in the case.
I don't like ANY of the plastic cases I've seen for my MBA and thus, am going "naked". If you're worried about scratches, you might take a look at a Zagg protective shield. I think they run about $50 for the MBA. However, I doubt the Zagg will protect against dents, but it should do fine against scratches.
And in truth, I haven't been very protective with my iPad wihich had been out of the house several times. If I had been, perhaps the few bottom scratches would not be there on it.