Index: wp-includes/category-template.php
===================================================================
--- wp-includes/category-template.php	(revision 17369)
+++ wp-includes/category-template.php	(working copy)
@@ -878,6 +878,7 @@
 				elseif ( $category->term_id == $_current_category->parent )
 					$class .=  ' current-cat-parent';
 			}
+			$class = implode( ' ', apply_filters( 'category_css_class', explode( ' ', $class ), $category ) );
 			$output .=  ' class="' . $class . '"';
 			$output .= ">$link\n";
 		} else {