Hi Jesper,
I apologize, you are correct - misread the question (top menu is what I was looking at) - but luckily the use of the image cleared it all up. I added the previous image so that you can compare it visually with what you see and help me see it as well. :) [also when I get support myself elsewhere, I often have a feeling that they say they looked, but in reality have done nothing at all, and so I like to add screenshots so no one would think that of me.]
There are jQuery errors on the dropdown. Please follow the instructions here as to how to resolve it http://www.kriesi.at/support/topic/firebug-error-product-variations
But... that won't fix the problem with IE. After you see in Firebug that the error is gone, please add this directly to your header.php file., right before the </head> tag (the closing one).
<!--[if IE]>
<style type='text/css' id='variations-select-correction'>
#top .variations select{min-width: 100%}
</style>
<![endif]-->
Please don't forget that when you update the theme, this will be overwritten and you will need to add it again. (unless you've made a child theme)
Thanks!
Nick