I'd probably hang onto it myself, but wouldn't give it away/sell it. Use it in another room or something. I have this happen all the time with hardware vendors... an extra laptop comes back or something. I almost aways call them and ship it back - but on a 360 I had already had warranty issues with... I dunno. The only time I've kept something was a spare LCD monitor, but I had also sent the thing back 4 times for repairs.

The "extra" one died a year later anyway