It's also worth noting that ZFS has been available as a root file system long before FreeBSD added ZFS to RELEASE. I remember running OpenSolaris (and some of it's forks, eg Nexenta) with ZFS root about 6 years ago. Possibly longer actually.
The issue with ZFS as root was more of a problem with the boot menu than the OS. OpenSolaris used GRUB where as FreeBSD obviously doesn't, so FreeBSD needs to either port their ZFS drivers to their bootloader, or employ a hacky method of having a UFS boot volume that then points to a ZFS root partition (which, sadly, is how FreeBSD currently works).
Interestingly, since GRUB is GPL, it meant that technically there were GPL ZFS drivers even before Btrfs started life (never mind the various Linux ports of CDDL-licenced ZFS drivers that have appeared since). Albeit those GPL ZFS drivers were read only
A ZFS-aware loader hit CURRENT in late 2008, and a dedicated zfsloader for use from (gpt)zfsboot hit the stable branches in late 2009.
Anyhow, thank you for the correction :)