The Air is quite a bit tougher than most people seem to give it credit for. It certainly looks like a frail little thing, but it can withstand quite a bit with no ill effects. Here is how my Air has traveled around for the last year now, and it still looks nearly new. No case, no invisible shield, none of that is to be found on it. I considered getting something for it, but then I thought "I didn't get a light, thin, beautiful laptop to make it heavier, thicker, and uglier."
You sir, are my hero! lol...way to use your computer, not worry about it, and keep it looking the way it was designed. That being said, I do use at minimum a neoprene sleeve to carry my Air around in.
I also thought the OP's question was a good one. I sometime carry quite heavy loads in my backpack when going to work. Often times running to catch my train, and when I do the bag obviously bounces around hitting my back as I run. I'd be a bit worried at that kind of weight smashing into my Air.