If not compatible with CCK 6.x 2.0rc8

The solution by txcrew work: http://drupal.org/node/309367#comment-1021986
Only need to edit file: comment_cck.module

txcrew say:

After further investigation I believe to have tracked down the line to change. Please forgive my lack of programming knowledge.

I changed line #58 from

if ($form_id == '_content_admin_field') {

to

if ($form_id == 'content_field_edit_form') {

Hope this is a step in the right direction.

txcrew