Under the Parameters section of the drupal_add_css() function, the documentation says this:
$path (optional) The path to the CSS file relative to the base_path(), e.g., /modules/devel/devel.css.
I believe this is incorrect; the CSS file should not include a preceding slash. The example should instead be
$path (optional) The path to the CSS file relative to the base_path(), e.g., modules/devel/devel.css.
In the drupal_get_css() function, one can see that base_path() is actually prepended onto the filename. I'm looking specifically at one of the lines that looks like this:
$no_module_preprocess .= '<link type="text/css" rel="stylesheet" media="'. $media .'" href="'. base_path() . $file . $query_string .'" />'."\n";
I spent a few hours today trying to find out why my calls to drupal_add_css() weren't working, only to track it down to this. Hopefully this can be fixed soon, if this is indeed a bug.