Indeed it was the screenshot image that was pushing the sidebar to the bottom.
To anyone who stumbles across this thread and has had a problem with the sidebar jumping to the bottom of the page in IE:
In other browsers, if an image is uploaded to a site that is too large–say it is 600 px wide, but the WordPress column that it is sitting in is set at 500 px–then the browser just shrinks the image appropriately and it looks clean. But IE screws it up and crowds whatever else you might have in that horizontal space, and the sidebar falls to the bottom.
Unfortunately, and I don’t quite understand this, IE can also drop the sidebar if your total page width is not set wide enough. This problem seems to be independent of any images you might have in your posts. I had originally set my total page width to 812 px, which was enough space to hold my main column and sidebar in all other browsers. But it wasn’t until I added a few extra px to this setting (up to 827 px) that IE was able to fit both the main column and the sidebar in properly. Others may be able to explain this better and there are plenty of other threads discussing this for anyone who is having trouble.