You will have no issues using the EF100-400mm lens on the 40D. The 1D bodies have better weather sealing and more robust shutters, but they don't inherently work better with the "L" series lenses. You will have some differences in effective focal length because the 40D sensor is not full frame like you would find on the 1DsMKIII or the 5D MKII, but you may find this to your advantage as it makes the lens appear even longer.


There is a discussion about crop factor in another active thread on this board if you want to know more about sensor size and focal length.