I think it's just dust on your sensor and it needs to be cleaned, that's all. Most of the time I've had dust on the sensor, it doesn't move - but sometimes it will. The mirror moving up and down inside the mirror box is apparently a powerful enough "fan" to do blow it around. Cleaning the sensor takes care of 95% of the problem but if there's dust inside the mirror box (stuck to the black surfaces around the sides) then it'll just move around and wind up on the sensor too.

One friend uses a miniature vacuum cleaner attachment for cleaning electronic stuff on the insides of his camera once in awhile, but I'm not quite that... particular... I think Oreck (the hotel vac people) make it.