I saw other CSer's airshow photo also have the same probelms. In normal case we do shoot vertical up, if there is any dirt in the camera, it will fall right on the sensor.
During airshow, I shoot with two camera 350D & 20D, 350D had no problems but for 20D after shooting about 200 shots the dirt spot start to appear. at the end of day after using a blower and blow into the sensor, the dirt spot disappear. The problem appear three time (day), at the end of day blow clean the sensor and the dirt spot disappear again. My 20D just return from CSC for sensor cleaning one month ago. Look like I need to do sensor cleaning everytime after shooting.