Ron,
From what I've heard, the ability to create a null whitelist entry is for ease of troubleshooting. It allows one to retain the configuration for a whitelist entry without having to keep the IP whitelisted. This is helpful, for instance, if we want to temporarily disable a whitelist entry without having to completely recreate it. Thanks for keeping your eye out for interesting occurrences like this one!
Jorel Haggard
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com