Some time ago, I had <a href="../../node/12">issues</a> when enabling CSS optimisation using the <a href="http://drupal.org/project/deco">Deco</a> theme (which is also used by this site as well). I've solved the issue quite some time ago, but never got around to actually presenting the solution.
The solution I found was on the Deco bugtracker (available on Drupal site), but I can't recall anymore what the link was. In hope of helping other people using this nice and stylish theme for their own Drupal deployments, I've posted the patch for the Deco theme here, as well as complete files which should be copied over the existing files in the Deco theme directory (whichever way you prefer).
I should probably try poking the Deco theme maintainer to fix this and make a new release as well, but that'll have to wait until I've got more time.