Although your problem does not show up in my browser (Safari) I struggled with a similar problem the other day, and found that if I put a “vertical-align: top” onto the container that holds both the left content and the right sidebar, the problem disappeared.
Skimming through your css, I didn’t see any vertical-align: top, but maybe I overlooked it.
Most likely it happens only in IE… and a search for “sidebar down IE” would give you plenty of fixes…