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Its one year warranty has nothing to do with additional phone purchases; it's covered until 07/11/09. Although, to get a replacement, they usually requrie it has an active line, which may require throwing your new SIM in it and re-activating it temporarily; depends on the Genuis/Manager.
 
Did this yesterday. Take your SIM out of your 3G S and put it in your 3G. Take it to Apple, they'll switch it out. Walk out with your "brand new" refurb. I'm using my old 3G as a back up/jailbroken phone. Redsn0w'ed it the second I got home with it :)
 
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