Install a FreeBSD 9 system with zfs root using the new installer:

Start the install, drop to shell when it asks about disks, run these commands:

# this first command assumes there has never been anything on the disk,
# you may need to "gpart delete" some things first
# also assumes there's nothing on the disk you want, buyer beware, etc.
# don't blame me if you lose your data
gpart create -s gpt ada0
gpart add -s 64K -t freebsd-boot ada0
# adjust swap size to taste
gpart add -s 4G -t freebsd-swap -l swap0 ada0
gpart add -t freebsd-zfs -l disk0 ada0
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ada0
mdconfig -a -t swap -s 16M
newfs /dev/md2
mount /dev/md2 /boot/zfs
sysctl kern.geom.debugflags=0x10
zpool create zroot ada0p3
zfs set mountpoint=/mnt zroot
zfs mount zroot
exit

Finish install, select "Live CD" when installation is complete.

login as root, no password
cp /boot/zfs/zpool.cache /mnt/boot/zfs
echo 'zfs_load="YES"' > /mnt/boot/loader.conf
echo 'vfs.root.mountfrom="zfs:zroot"' >> /mnt/boot/loader.conf
echo 'zfs_enable="YES"' >> /mnt/etc/rc.conf

cat << EOF > /mnt/etc/fstab
/dev/gpt/swap0         none            swap    sw           0       0
EOF

zfs unmount -a
zfs set mountpoint=legacy zroot
zpool set bootfs=zroot zroot

reboot

# NOTE:

# there may be better ways, see:

# https://wiki.freebsd.org/RootOnZFS
# http://www.aisecure.net/2012/01/16/rootzfs/
# http://forums.freebsd.org/showthread.php?t=31662