It's probably still got a small amount of water in it.
I suggest placing your phone under a gentle heat source i.e. 60 watt lamp, over night, making sure the bulb is about 8 inches apart from your phone. I saw this method on the gadget show.
Alternatively, place the phone in your airing cupboard for a week. This fixed the same problem for me.
Alan Sargeant, February 2009