You can return it but likely won't walk out with a new one. I thought I could swap mine with some of the replacement stock they have but no luck. The genius said he would be happy to swap it out if they had new ones in stock. Only option was to return it and try to go through the reservation system for a new one. I decided to keep mine since I don't have a backup phone to use.
Try the suggestion I posted here (it's a bit lengthy). Your situation is even easier, as you bought yours from Apple, so you don't have to do the whole AT&T to Apple to AT&T thing as I did. If you have another line that had a potential upgrade on it, it's even easier still, as you don't have to do the whole new line thing I was recommending to the Original Poster. Just keep in mind the caveat about the money.