I accidentally deleted the message from the guy who responded, but he got me on the right track and I wanted to acknowledge it. In case anyone comes across the same problem, I thought I'd pass along my fix.
I did, indeed, have the server set to send .css files as test/css. Unfortunately, I also had apache set up to parse all .css files through the PHP engine for various reasons. This had the effect of overriding the AddType directive. The fix I applied was to add the following statement to the very top of the problem CSS file
<?php header('Content-type: text/css') ?>