Having just got an early upgrade to a 4GS, my iPhone 3GS is surplus to requirements. It's the 16Gb version and is in perfect working order and in excellent condition. It's boxed, with a black silicone case and complete with everything it originally came with. It's currently locked to Orange but these are easily unlocked for about a tenner.
£200 delivered (UK mainland). Paypal as a gift or direct transfer, sorry no cheques.