If your first question is ‘why not linux?’ is that I truly think Solaris is a better and more mature operating system. Solaris has native ZFS wich is still a major issue with the GNU part of linux. Solaris is also very ready for what you expect from a NAS and you can download it for free. Solaris has nfs, cifs, iscsi, snapshotting, raid protection directly built into it. I’m also a big Mac user so I also installed the mdns responder (howl) so now my Solaris server nicely shows up in the Finder and I can see my AFP shares which are protected with ZFS selfhealing.
The following link helped me configuring it:
http://www.tek-ops.com/archives/20
This link helped me further understanding the user management and setting up the right permissions on the directories:
http://users.phg-online.de/tk/netatalk/doc/test-afpd/
[update 2009/march/01] here a link howto compile on opensolaris
http://darkdust.net/writings/opensolaris/compilingnetatalkonopensolaris
