I also had a similar damage to my apple logo on my Macbook air, except the white plastic was not crack, but loose and barely attached to the notebook casing..I was surprise for such an expensive hardware, they only use tape to attach the apple logo to the aluminum casing. At first I thought about leaving it alone, since it was only cosmetic damage, but the more I thought about it, the more annoy I got...eventually I took it to the Apple store and had them check it over, within 20 mins, I walked out with a brand new Macbook air (I didn't have a receipt because the Macbook Air was purchased on eBay, but the Apple employee check the warranty and notice it was only about three weeks old, so he offer to exchange it). I would definitely take it to an Apple store or call them up, if the Macbook Air is less than two weeks (or in my case three) old, they should exchange it for free, if you had it longer than that, they will probably repair it.