http://example.com/my-post-permalink/
http://example.com/my-post-permalink/comment-page-1/
http://example.com/my-post-permalink/comment-page-2/

My solution in this short plugin is to have all but the main permalink show an excerpt of the post, with a link back to the permalink page.

<?php

/* 
 Plugin Name: SEO for Paged Comments
 Version: 1.1
 Description: Reduce SEO problems when using WordPress's paged comments.
 Author: Austin Matzko
 Author URI: http://www.pressedwords.com
 */

function seo_paged_comments_content_filter($t = '') {
	if ( function_exists('get_query_var') ) {
		$cpage = intval(get_query_var('cpage'));
		if ( ! empty( $cpage ) ) {
			remove_filter('the_content', 'seo_paged_comments_content_filter');
			$t = get_the_excerpt();
			$t .= sprintf('<p><a href="%1$s">%2$s</a></p>', get_permalink(), get_the_title());
		}
	}
	return $t;
}

add_filter('the_content', 'seo_paged_comments_content_filter');

Download:
seo-for-paged-comments.zip

[Updated January 30, 2009]