Thanks for pointing it out. This appears to be a bug in NeatHtml which we use to protect against cross site scripting in untrusted content like forums posts. It seems that NeatHtml is rendering some markup withing IE 6 comments so it only affects IE 6. When I view the rendered source of the page I see:
<!--[if gte IE 7]><!-->
<div class='NeatHtml' style='overflow: hidden; position: relative; border: none; padding: 0; margin: 0;'>
<!--[if lt IE 7]>
<div class='NeatHtml' style='width: 100%; height: 400px; overflow: auto; position: relative; border: none; padding: 0; margin: 0;'>
so it seems this height:400px; is the cause of the problem, when I remove NeatHtml from the equation the problem goes away. I'm not sure why that would be needed, I will follow up with Dean Brettle, the author of NeatHtml.