I Guess its going to get talked about in the next installment of this, but as I understand it, MacOS X can use its own NetInfo, LDAP, AD, and a few other things to get directory information to manage all sorts of things.
I recently set up Posffix and Courier-IMAP with virtual accounts stored in LDAP. After seeing so many different poorly documented ways to do it, I finally went with a toll called Jamm because they documented the setup well and provided the needed management user interface so I can deligate account management to others who aren't sys admins.
For better and often worse, Microsoft did clear up the tower of babel of different LDAP schemas and tools to make one cohesive system called AD. But Apple has also done something similar for its platform in a more open flexible way.
The beauty of UNIX is you can do things very a-la-carte, picking what is best for you and integrating it together yourself. But there are also lots of people who want it to just work and want someone else to have already figured that bit out. There's room for both.
I think AD support for Linux is a good answer for helping it integrate into exsisting enterprises, but Linux (or the distributions) should come up with an alternative, open, cohesive standard for doing the same things.