what I wake up to
I just cleaned the sensor on my new 20D. it was filthy. (because I got it well used) like covered with grease and such. no wonder I've never had a problem with this on my own cameras, because I don't leave the lens off and eat cheeseburgers while hanging over the camera.
it wasn't too difficult, other than taking a while. you can't really scrub or use detergents. so lots and lots of careful swiping with a cloth and ethanol.