I never understood the desire people have for a second body. Unless you're a pro, why waste your money? If you want the 5D more, get it and sell your 40D for the 35mm.
That's unfortunate for you to say. Not everyone shoots like you or sees photography the same way you do.
a) Backup. What's wrong with that? Some hobbyists take this very seriously.
b) Different tools for different purposes. I'm a big fan of the fullframe/crop combination. I think a 5D, 1D, and 40D would be a killer combo and not a waste at all. 5D for landscapes, portraits, low-light, macro, and most things. 1D for heavy duty, fast paced work. 40D for some extra reach for things like birding.
c) It's a lot nicer to have 2 bodies on a shoot than 1. Try it some day, you'll understand.