IE stylesheets defined in Themes don't get served with IE conditional comments

Theme stylesheets get served by /styles/combined.css as CSS imports. This doesn't work very well when you have an IE stylesheet defined e.g.

<resource name="fixedwidth-ie.css" type="download" location="fixedwidth/css/fixedwidth-ie.css">
          <param name="ieonly" value="true"/>
 </resource>

It always gets included in the combined.css request.