Is the outer shell actually metal on the 40D?
From what I have read the frame is made from stainless steel and some magnesium alloy (can't find anything that says what the actual alloy is), while the outer shell is plastic and rubber. I don't have a 40D, but every single review I read says the same thing.
Here's one. Go down to the "Look and Feel" section, about 1/3 down.
I think just taking the body apart would be enough to prevent most people from attempting this. Assuming that the outer shell was in fact magnesium, you would have to completely disassemble the camera, including taking off the rubber grips.
If it were
me, I would just mask off the important areas then airbrush it. It would look almost as good as a good anodizing, but be much easier.