Wow, this went crap. My iPhone has a huge crack on the back, which is a well known problem with the 3Gs. Also, the auto brightness sensor doesn't work and the screen has a few dead pixels.
Took it to a 'genius', showed him the crack, straight away he said "I'll go and get you a new phone, this is a very common problem". Came back and had a little light to look in the headphone jack and dock connector, and apparently my phone has been in contact with water
This is what he was looking at. It has never been in contact with water as far as I know. He spoke to the 'head geniuses' and the manager and they said no I can't have a replacement or have it fixed. So now i'm stuck with a crap iPhone that probably has no resell value what so ever.
He said I can either buy a new one for £390 or claim it on house insurance.