Thanks for all the replies folks. I don't think I made my question clear, though. There are lots of ways to deny someone access to a system rather than just using the /etc/passwd file, and most of them came out in this thread (ie, setting the shell to a dummy shell, using the passwd binary, etc). I guess what I really wanted to know is if someone knew of a document listing the different characters that appear in /etc/passwd files and what they meant, but it seems from the different posts here that the only common character used in the /etc/passwd file for all the *nix flavors is the "*". All others seem to be unique to each *nix flavor.

zen -- You may be right about the sshd daemon not parsing the /etc/passwd file on Linux, but it does seem to pay attention to that file on HP-UX. When that password is set to * on HP-UX, ssh will NOT let me log in as an authenticated public-key user. As soon as that password is set to something else, however, I can slide right in. For Linux, however, the /etc/passwd file does appear to be completely ignored. Unfortunately, the "documentation" for OpenSSH amounts to 6-7 man pages. You would think there would be more info about this protocol by now.

Thanks everyone for their posts!