trackbacks to posts never show up in the blog, but they are in the databse. Looking at wordpress_comments you can see all trackbacks have a post_ID of "0" and almost all other values are empty.
The reason for that is wp-trackback.php and wp-includes/functions-post.php make inconsistant use of variables. wp-trackback.php uses comment_post_id, wp-includes/functions-post.php uses both comment_post_id and comment_post_ID. The final insert into the database uses comment_post_ID, and so does wp-comments-post.php. That is why comments get written into database with the correct ID and trackbacks with an ID of "0".
While the variables comment_author, comment_author_email, comment_author_url, comment_content, comment_type still have the correct values in wp-trackback.php those values get lost in wp-includes/functions-post.php. this only happens for trackbacks and not for comments.
And btw.: wp-trackback.php uses commment_author (with 3 m) instead of comment_author.
