IE is often weird, but none
I suppose there could be an oddity with the stylesheet syntax, or with the way it is linked to the page. Are you defining a DOCTYPE on your page, and specifying the meta-data as shown below?
<link rel="stylesheet" type="text/css" href="file.css">
Yeah, I guess you are, or it wouldn't validate.
We can only take wild guesses without looking at some code or something.