You must remember a couple things.
(1) the center column can vary in size - compare the center column on a blog page with 10 full posts vrs a single post page
(2) the center column can have multiple posts and each post is in a container that rest atop the center column, commonly, these post containers are seperated by some degree so there would be a gap between them
the problems that come up are how big to make a background image for the center column and how to repeat it. Remember that not only can the height of the center colum change, but if you are not using a fixed width page size, the width of the center column can change.
to see what I mean, add the following to ATO->Center->Center column style
and look at how it shows up on several of your pages.