Nice clean layout (including the HTML source).
When viewing in Firefox, a bug shows up in that your main content area doesn’t display the background all the way down for the pages with more content than can display in the browser window at one time. If you’re viewing with a large resolution you won’t notice, but on a smaller res (800x600, or even 1024x768 on the longer content pages) - defining something with height of 100% causes it (in FF, at least) to be 100% of the original screen height, so when you scroll down, the background color and ‘body_back.jpg’ image stop.
It looks like you can work around this by repeating the background-color, background-image stuff inside the child divs as well as in the body div. You’ll then want to get rid of the couple ‘margin-top, margin-bottom’ attributes and replace them with ‘padding-top, padding-bottom’ so as not to cause a gap between the areas.
I notice in IE, for short pages, the image just stops at the end of the content and ignores the 100% altogether. Not sure what you can do there, you’d probably have to experiment.
Another personal taste thing, I’d add ‘padding-bottom: 1em;’ into your css for the footer div. Text ending right at the browser window seems too abrupt, like printing right to the edge of the paper. You need a little bit of air space (again, something you notice on a smaller resolution, as you won’t see it if browsing with a 1200px screen height - given the length of the content currently on the pages).