Its an annoying trait of some Nikons too. May have something to do with the sensors (both co's use Sony)
This artifact, when it occurs, is particularly noticable at high ISO numbers and dark backgrounds. All sorts of theories abound. Try over on
www.fredmiranda.com. earch for "banding" & "EOS 40D".
If its still under warranty send it back to Canon for a tune-up.
Good luck.