If you are in the need for lots of cheap storage and cannot afford off the shelf NAS solutions, this is definitely an option to look at.   These units provide an external box with 4-8 drive bays that connects to a PC via e-sata interface.    The e-sata interface must be able to support a port multiplier option for this to work and the units usually come with a dual port PCI-E e-sata controller.   Sans Digital is not the only manufacturer/supplier of this kind of unit.

We have tested these units using various debian flavored operating systems and they do work well using mdadm for RAID management and setting the controller just to pass through the drives.  We even use cryptsetup for fully encrypted volumes.  However there are some caveats. 

• Stay away from WD20EARS drives.  Unless you disable WDIDLE, the drives will randomly drop out of your array when the green functions kick in and make the drives sleep.  We prefer the 7k Hitatchi desktop drives.
• Don't use controllers with the Marvell 88SE1928 chipset like the bundled Highpoint RocketRaid 622.  The Linux driver is poor and under load there are plenty of hpt_resets that will pause file transfer or make it crap out altogether
• Do use any controller with the Silicon Image 3123 chipset.  Although not very fast, it is rock solid and well supported under linux
• Running cryptsetup on your RAID volume slows it down quite a bit, at least 30% in our estimates.   We haven't been able to figure out how to improve this. 

For a cheap NAS front end, checkout Newegg for their Foxconn barebone units.  You can use 2 e-sata controller cards and plug 2x 8-bay units for a total of 3TB x16 = 48TB. The Foxconn + CPU + RAM + Disk will all come in at under $200 shipped.  Each 8-bay unit is around $350 shipped and the drives can be had for under $200 each.  So the total price will come in at around $4100 at max configuration.   If you did decide to do that,  I would recommend using a 2nd Foxconn and adding a dual Intel based gigabit network card as the Foxconn network speeds are not too impressive.  This will also allow you to use bonding to increase the throughput of the NAS.  Those cost around $200 each so for the 2 NAS 48TB setup it will cost $4700 or about $100/Terabyte.