wp-comments-post.php does not replace wp-comment.php. In WordPress 1.5, the code that was in the previous wp-comment.php has moved into the file comments.php in your theme directory (beneath wp-content/themes).
You can see my blog entry on wp-comments.php for more details:
http://equickfixes.com/2006/01/08/where-did-wp-commentsphp-go/
