It seems that if you edit a post and removed the subject that it does not do a check even though it is marked as *required so you could actually go back and remove the subject then nothing would appear.
I had discovered it by mistake when I went to edit a post and must have tab over the subject and erased it. Then when I viewed the post again the subject was gone. When you edit it is marked with a *required but my guess is the check for the required fields is missing in the edit_post.asp file is missing.
Here is the test I did on Web Wiz: http://forums.webwiz.net/forum_posts.asp?TID=5025&PN=1