I got my Xbox 360 back from Microsoft yesterday. I forgot to check the serial number to see if I got the same unit back, or a different one. I will check tonight.
They were pretty fast; it only took about 9 business days for me to receive it, including all shipping time. UPS shipping was free, by the way.
When the UPS guy gave me the box, he said "Gettin your Xbox back?" (It was shipped in a featureless white box) I said "Yeah, you\'ve seen these before?" He replied, "Yeah, I\'ve seen a lot of em. That\'s why I decided not to buy one."