Why would you want to do that? The trade-in for iPods is only 10%, right? If you trade in a 3G for 10%, that would only be $70 at the highest price point.
You could sell it for way more than your discount would be.
Send it to envirofone or similar - if you're not wanting to sell. Your carrier won't give you a trade in price, so may as well get £200 or £300 odd bucks for it (16GB as an example).