Found this thread on a 20D problem. The thread describes a problem with 20D sensors that may be caused by manufacturing defects with the sensor. It looks like dust on the sensor but is not removed by cleaning and is increasingly apparent when the lens is stopped down. I have checked my 20D and found the same problem, but the spots on the sensor aren't apparent in everyday use. As stated they are particularly noticeable if you take an image of an overcast sky and stop the lens down. Here's the link
http://forums.photographysites.com/forum9/78.html and it makes interesting reading. This may be a quirk of all digital cameras, I would be interested to know if other members (with canon and other brands of camera) have the same blotches apparent under the test conditions described. I have attached a very compressed image with the offending spots circled to indicate the problem. The dark patches are much more apparent in the original image.