I've been playing around with the CSS for the site, and found an annotated WP layout file on Nuclear Moose's website. It's a handy reference, but I think it would be more useful if the commentary were actually enclosed in CSS comments. To that end, I've copied the relevent text and enclosed the explanations in comments. Here's my commented version. Hope this helps someone get a handle on layout in WP (I know it's taking me long enough).