Looks like a missing closing quote for src tag.
original
Code:
<iframe src="http://www.facebook.com/widgets/like.php?href=http://www.YOURSITE.com/showthread.php$session[sessionurl]?p=$post[postid]&show_faces=false&
scrolling="no" frameborder="0"
style="border:none; width:450px; height:25px"></iframe>
closing quote added
Code:
<iframe src="http://www.facebook.com/widgets/like.php?href=http://www.YOURSITE.com/showthread.php$session[sessionurl]?p=$post[postid]&show_faces=false&"
scrolling="no" frameborder="0"
style="border:none; width:450px; height:25px"></iframe>
I haven't tried this yet but seems some browsers may have an issue with this.