Do you know IE has a limit of 32 external CSS files? Do you know each file should be up to 288Kb to get parsed?
I didn’t know and found this just a little bit of time ago. As Google says IE has a bug (well, someone names this a limitation which makes sense): number of external CSS files you can import is 32, everything beyond the 32th file will be ignored. In addition to this – CSS file size cannot exceed 288Kb, otherwise it will be ignored as well. This limit is present in Internet Explorer 6/7 and 8.
Seems IE is the only browser with such a limit, other browsers worked well for me with 33 external style sheets (Firefox, Opera, Safari, Google Chrome, Konqueror).
Solution: aggregate multiple CSS files in bigger ones. As my experience shows – most files are 5-20Kb in size, and one 40Kb file is faster than 8x5Kb.