IE and Preprocessed Sub Regions

I'm using the code found here http://drupal.org/node/1230110#comment-4834770 to create 2 sub content columns below the main content region. It works well except I noticed in IE 8 and below, my right sidebar floats below them instead of along side. I've attached a screenshot, but you can see the code here. the two sub regions are Preface One and Preface Two:

http://rosewebdesign.com/mirc

Can anyone suggest how I can get this working in IE 8 and lower?

thanks.