The content frame (the part between the top and bottom rounded corners which DO display) won’t display a background on IE. On Firefox, Safari it works as expected.
My guess is that, for some reason, IE thinks the height of the content area is zero, and therefore not wanting to display a background. Setting height: 100px (or any other fixed value) doesn’t help. To ensure I had things right, I tried "background: red" and sure enough, the whole area turned red. I also added a 1px border, which bumped the sidebar down under the main area, and IE did then render the content’s background.
I have to admit I design in Firefox and then test in other browsers. I’m also a programmer, not a CSS guru. Any help here would be appreciated. I used to care about doing things right rather than doing them as a hack, but right now I’d accept a hack. :)