The problem seems to be that none of the containers on the forum (<div> and <span>) are fixed width. IE doesn't seem to understand that 100% is the width of the browser window.
I'll look into this more when I get home from work.
I don't think that is quite the issue. The code tags make the width greater than 100%. Now... if that's not supposed to happen and you're supposed to just scroll horizontally in code tags (it made it over 100% in the old forum format as well), then that's an issue with the code tag format. But, if the code tags are supposed to be allowed to increase the width of the messages, then it's an issue with the left "cell" that contains the poster names.
Here is what I think is happening.
Poster Cell = 20% (example... I didn't try to find out what it really is)
Message Cell = 100% (this makes the message cell the remaining width to fit the window).
That would work fine... until the code tag increases the width of the table. If the table is suddenly 200% because of a code tag, then that poster cell is 2 times as large as it was originally because 20% of 200% is bigger than 20% of 100%.
The fix that I think would be best would be to set the poster name cell to a fixed width instead of a percentage. The other option that I can see, though I wouldn't like it as much, is to prevent the code tags from being wider than the screen size and make users scroll horizontally on long lines.