"custom comment types" won't work

twentyten only supports the comment-types ”, ‘pingback’ and ‘trackback’ any other type won’t be displayed!

rearranging the switch/cases should fix the problem:

switch ( $comment->comment_type ) :
  case 'pingback'  :
  case 'trackback' :
    some code...
    break;
  case '' :
  default :
    some code...
    break;