I've been working on this all night!
It appears that IE is rendering a space above the "primary" and "content" divs on this page:
In Mozilla / Firefox, everything appears to be fine. But in IE, I can't get the spaces to go away. I realize there are some markup errors, but these aren't causing the problem (I've already tried to remove the code in question).
This is most of the stylesheet:
[CSS code removed - a link to your site is enough to access the stylesheet]