I've read about this that you would have to fool WordPress into thinking that you are on a single post to get the comments template and have found a way to do that.
as Jan Fabry pointed out you need to mind the $POST->ID, so you will need to create a stub post for each category or tag , just for the ID so that post will hold all comments.
the code was updated.
in your themes archive.php or category.php just after the loop paste this code:
//save the true post id
$true_id = $post->ID;
// populate $post with the stub post
$wp_query->is_single = true;
//reset wordpress to ture post
$wp_query->is_single = false;
you will need to change the STUB_POST_ID with the stub post id!
and you can use a conditional tag to change to the right stub id like this:
$post->ID = 25;
$post->ID = 30;
weird but it works :)