I've thought about it. I'd say it boils down to the following important points:
Simplest method is to have the cameras dump their feed via FTP
Trickier but more elegant would be a Windows/whatever OS the management software runs on machine doing the cool stuff and using FreeNAS as storage. This tends to imply iSCSI, unfortunately.
Other thoughts:
ZFS might be overkill for CCTV - or it might not, if you're paranoid.
Hardware requirements could be lowered with a simpler solution - however, hardware RAID tends to creep forward when we're talking about massive arrays. At that point, you might as well use ZFS.
Fragmentation could be a complicated issue.
Hardware must be sized to allow for scrubs/resilvers during normal operation without impacting the streams.