I've never sent back my 360. I bought one, the disc drive was shoddy, I had it replaced at BestBuy in-store at no cost and kept my hard drive. That was 40 minutes of anguish, let me tell you.
The disc damage was so minor that it took like 4 months for me to notice the marks. I did it to be safe. 0 issues with replacement.
I've always kept my 360 in totally open air, on top of an entertainment center or on a desk or what have you, never enclosed.