2 Questions:
Does IE eventually load with the newer stylesheet?
How are you saving the changes to the server (SSH, Web Interface, etc.)?
I ask because I ran into an issue with a web-based file management system a while back where one browser would show the CSS just fine, the other wouldn't, and we noticed the issue of which browser played nicely depended on which browser we used to upload the CSS.
The reason turned out to be that IE gave the CSS file a different MIME type than Firefox. If you continue to notice the issue even when you've cleared the cache, check the MIME type of the file using Firefox by going to Tools > Page Info.
Also, if it is simply a caching issue, consider turning caching off in IE. I believe this should be an Advanced Option under Internet Options.
Also, you may want to try creating a special developer environment for IE (if it comes down to it.) Perhaps have another machine with IE7 and no caching and any other features turned off (A kiosk mode, perhaps), and then previewing the page on that machine.