Perhaps certain post subjects are just on the verge of the limit, and when RE: is prefixed onto the subject name it prevents the post being registered due to a limitation of the subject field's length in the forum database.
Perhaps only allow a subject length of 80 on a forum post (ie, make the limit occur in the text field of subject when making a post) and when people reply it will not go over the database limit of 85.
Also John, just so you can verify the problem exists, try post a quick reply (ie, don't click edit but just use the text box down the bottom of the page to reply) to this thread:
http://forum.toplace.com/showthread.php?tid=5272
I think clicking reply allows it due to some formatting that the reply page does with the subject. (As you can see in the thread I linked, it gets rid of some of the starting characters of the subject to make it fit inside the limit)