Hi David,
On your site this problem appears to be IE specific, it looks fine in Firefox.
But I have a solution for you because I ran into this issue recently myself. After the last release I added a data point to the users table to show total revenue from each user from any commerce enabled feature and then I addedit into the forums on the left side with the other user info. I did this because it was starting to become difficult to remember who my customers are, who bought me a beer etc. I mean I strive to give everyone good support but hey if post comes in from someone who has bought a product or bought me a beer I'm more likely to answer it after working hours. So anyway when I added this extra data to the left of the forums to show user revenue (only to authorized roles) it caused this same layout problem in the forums for me in all browsers (the same as what you have in IE) in all the included skins so I had to fix them as follows. Basically you should add what I have in the curly braces {} to the same css classes in your forummodule.css file for your skin. Leave the settings you have there and add these:
.postwrapper {float: left; width:100%; }
.postleft {clear: left; }
.postbody { min-height:80px; }
Then clear the server cache by touching Web.config, then clear the browser cache of temp internet files.
Hope it helps,
Joe