Drupal aggregate css crashes IE7
There is a size limit of 300kb per file and a limit of 32 files.

http://joshua.perina.com/africa/gambia/fajara/post/internet-explorer-css-file-size-limit

http://social.msdn.microsoft.com/Forums/en-US/iewebdevelopment/thread/ad1b6e88-bbfa-4cc4-9e95-3889b82a7c1d