I'm also using the Fadtastic theme and have two screen grabs of its comment box rendered with IE7 on a 1024x768 monitor at full-screen resolution.
I think the problem occurs on the individual post pages when the content column on the left is shorter than the adjacent sidebar.
Here's what the comments box looks like when the content extends past the adjacent sidebar:
But when the post doesn't extend past the sidebar the comment box is improperly rendered:
Of course in a perfect world everyone would be using Firefox, but then life would be too easy ;-)