I just tried out the free download of Openfiler (
http://www.openfiler.com) not long ago. Openfiler is built on top of CentOS. It supports pretty much everything you'd expect a NAS to; CIFS, NFS, software RAID, iSCSI, etc. It was painless install. However, they expect you to pay for their admin guide, but some quick Google searches will give you all the info you need to get it setup. My playing around with it involved setting up a software raid and exporting some NFS shares to my linux boxes and joining Openfiler to an Active Directory domain. If you're looking to get something setup quickly and fairly easily, it may be an option for you.
Even though I've been testing out some of the more "turnkey" solutions, building your own NAS on top of your linux distro of choice is probably the most fun and rewarding route to take. It's a great way to start learning or brush up on your Samba knowledge. I'm doing the same thing right now with good ol' Slacky. And then if you miss having a web interface for management just stick something like Webmin on the machine.
Best of luck finding the solution that's just right for you