I had a Macbook replaced after one week of purchase. It was actually right when the uMB released...
The techie took my compy, and brought out a new retail boxed one and handed me the box... I got to keep all new everything!
What did stupid me do? Brought my original retail box/charger and OFFERED them to him. Grrrrr... I could have had a second charger kickin' around. Given that I take my MB everywhere, it would have been nice to have one ALWAYS plugged in at my desk and one ALWAYS in my backpack...
I would chance keeping it. I doubt Apple expects/wants it back. They wouldn't have asked for mine had I not offered it up. -_-