Does anyone know what "!!" represents in the password field of the /etc/shadow file? 
no password set!!
I would suggest that you look in the man pages for the system on which you're running. It's always a good place to start debugging activity.
Would that also be the same as having "*" in the password field?
No it wouldn't; that means that shadow passwords are being used.
T,
The "*", asterisk, is used a as general "wildcard" placeholder in UNIX/LINX,
if placed in an /etc/passwd file in place of the password entry it would allow any password to gain access to that account. I'm am not so certain of its affect if placed into the /etc/shadow file, although I wouldn't be surprised if it had the same affect - give it a try and let us know.
Mr-Synapse
Am I looking at a shadow file that once was SCO Unix and now is Linux. It looks like those system accounts that existed previously all have a "" in the shadow file password field. New system accounts that were created for Linux have a "!!" in the shadow file password field. I am guessing that "" and "!!" have the same meaning.