good news
according to german c't magazin there is this board.
http://www.tragant.de/produkt/33079/merkmale/1.html
which is managing 10,3W idle.
Unfortunatly they dont say which haswell cpu they used.
an i5 for unraid is overkill.
the haswell celerons/pentiums are comming out in q4 2013.
for a fanless board with low power there is
http://www.msi.com/product/mb/C847MS-E33.html
that board runs idle at 13W with a pico psu here.
People have gotten the e350 down to 10 watt idle. That's by using ulv ram, pico psu and undervolting the CPU.
No one has achieved that with a ivy bridge yet. Maybe haswell when the celerons come out.
here is a link to ppl who build systems that use 10 watt while idle. mainly using e350 platforms. (its in german though)
http://www.hardwareluxx.de/community/f84/alltagstaugliche-desktop-systeme-mit-10w-idle-verbrauch-inkl-llano-fm1-beispiel-799083.html
there is a version of the celeron that is aimed at a nas setup.
http://www.slashgear.com/giada-n70e-dr-mini-itx-mainboard-aims-at-entry-level-nas-servers-11277397/
2x SATA 6Gb/s, 4x SATA 3Gb/s,
The box had Freenas with ZFS on it before, ZFS is nice but I don't like having all my disks spun up 24/7, maybe you can spin them down but I couldn't find a way.
use this script for freenas spindown on sas controllers
http://forums.freenas.org/showthread.php?1053-Unsure-of-SATA-drive-spindown&p=29123&viewfull=1#post29123
i tried the raid5 functionality and the write speed was terrible. something like 10 mb/sec.
that was on a i5 2500k with 8gb ram.
i now have 90mb/sec write speed to an encrypted zfs raidz pool.