I too am one of the many who is having problems with a falling sidebar. So others know, this is not a problem that is unique to WordPress. I had the exact same problem on another website I built. There were no orphaned div tags, no blog posts, no cms, just a plain html website. I could not resolve the problem and had to give up on CMS and move things into tables. I thought at the time it was a problem with IE7 because that is the only browser where I had the problem. Now I'm seeing the same thing with a new site in which I'm using WP and I'm seeing it in various browsers.
All of you are searching for something in your code, like a missing tag, but I think this is more of a problem with the browsers not obeying the rules. The reason I say that is because this problem comes and go when you refresh the page. That's why many of you have said, it was down at the bottom and now it's at the top and you don't know why. You refreshed; that's why.
But what that does or why, I haven't a clue. And I have no clue how to fix it. I gave up at one point and tried to put the content area of WP into a table, but then it all blew apart - no surprise. So I have to work with what was given me, but how do I fix this?
Here's the site: http://www.laurierking.com/wp.php You may see the sidebar there when you arrive. Refresh. If it doesn't go away, click the login link, then the "Back to Laurie R. King Mutterings" link at the bottom. That will usually do it.
If anyone can solve this problem, you no doubt will have oodles of good karma in the lives to come.