I just bought a sensor clean kit to clean my D40 in the last week. It was the first clean and the camera has done about 10000 shots. It really was quite bad. I put it down to buying a few more lenses in the last 12 months so I am changing lenses a lot more. Also a 4 week caravan trip into our Aussie outback last year did not help as red dust gets everywhere.
I am not sure if that helps you though!
I think you probably have a build up of dust floating around in the body so maybe you need to vacuum it out. This dust is falling back onto the sensor.The kit I bought had a little vacuum cleaner that came with it for this purpose.