UNIX Machines that don't use /etc/passwd

A Nameless Submmitor had this question to bring before you all: "I've heard of and had access to several machines that don't use passwd/shadow files or even wtmp for user logging/authentication, but instead use Oracle databases, or other methods. My question is, what's the story on this approach, what are the pros/cons, and is there anything out there for Linux that is similar?" There are SEVERAL options of this available for Linux, but just what do you all think about using a database for user authentication?