CSS problem in IE

I’ve got a design that renders perfectly in Firefox but has one quirk in IE. A huge gap appears between the bottom of the post and the beginning of the Comments section (happens on index.php and individual post pages). It looks like IE is pushing the Comment section down to make it start below the bottom of the left column/menu. I keep going over my CSS file but I can’t figure out why this is happening.
Site is http://olivia.hichens.name/wordpress/
CSS is at http://olivia.hichens.name/wordpress/wp-layout.css