IE fix for Omega theme?
As dddave mentioned it is probably because you have on your site more then 31 css files. Ie will use only 31 first files - other will be left alone.

What can you do with it? On live site - turn on standard drupal css aggregation. I will make creating theme painfull - you will need to clean cache(admin/settings/performance) after every css file change. So you can use the IE CSS Optimizer module:

Then there will be more option for css aggregation. It will make possible to aggregate all module's css, and having all theme's files included directly. Hopefully total number of files will be below 31.

For making easier adding condition styles for IE try the Conditional Stylesheets module: