To my understanding SCJD doesn't have version like SCJP. In fact saying SCJD 1.5 or 1.6 would be wrong I think cos SCJD (according to the exam code) has been out since
Java 1.2 or just 2. If you haven't registered for the assignment, when you download, it will have a dropdown menu saying "Java 1.1" or "Java 2" (same as 1.2). So 1.3, 1.4, 1.5, or 1.6 version of SCJD doesn't make sense.
So Sun recently put a limit of 1 year to do the assignment since Feb09. So if you are confident with say Swing, sockers/RMI, threading then do some practice before signing up for the assignment.