I had a similar experience a couple of weeks ago. I was apparantly doing 40 in a 30 area... again it is a bit misleading what the speed is in that area.
The policeman said I had been speeding and I said I had been unaware of that. He asked me if I lived nearby and when I told him I live just minutes away he said I should know the speed limit there. I replied I didn't say I didn't know the limit, I said I didn't know I was speeding! He was really nice about it and let me off with a warning! He even took the time to show me how far he could reach with his speed gun after I asked what the distance was on it. (Mental note to make sure I reduce speed well within range!)