A bit of contextual description:
I'm with Linux, so I don't use IE. The problem was reported to me with a screenshot. I upload it here for reference. The installation is at http://projectoria.org/forum/
I tested it then myself on Windows in an Internet cafe (so probably with high security restrictions, connection through a local proxy etc., if that would matter) and was able to reproduce it. I want to add that the <link> tag for the css was still present in the head and contained the correct path, but obviously style definitions didn't get loaded.
I am really not too involved in php scripting (or any more advanced programming), so I have no clue on what would have caused this. Could be, of course, an IE problem, rather than something with punBB.