I took my phone into my local CEX today and apart from having to wait three hours from leaving it with them, I am happy.
iPhone 3gs 32gb unlocked which they graded as B (only 'mint' is A): I got £216 vs the £177 which was the best offer today (15/05/11) using the tool I linked to above.
On the downside you do need to give them box/plug adaptor/USB lead/earphones, would have been nice to hang onto lead and adptor if you are considering another iPhone. I'd imagine they have some secondhand value as I think I saw earlier that Argos want a total of £46 for a genuine new USB lead and adaptor
If only I could have gone somewhere that I didn't spend anything during those three hours.....