If you are upgrading to an iPhone 4 then you may be selling your iPhone 3g S, i have done some digging around and find that cex offer the best cash prices for it,
also the added bonus that if you go into a store they will test it which takes an hour normally and then you get the cash in hand, if you opt for recycling or websites that buy your phone - you will have the hassle of sending it to them recorded delivery, waiting for them to test it and then give you final evaluation of price depending on condition etc, so alot of hassle.... cex i believe offer the easiest and fastest solution and the prices are the best i can see also.
-an unlocked iPhone 3g s 16gb in mint condition = £270 cash
-an unlocked iPhone 3g s 16gb in good condition = £240 cash
-an o2 locked iPhone 3g s 16gb in mint condition = £234 cash
-an o2 locked iPhone 3g s 16gb in good condition = £213 cash
***prices change all the time but these were correct at time of posting***