zfsboot support if currently only available in FreeBSD current as of late November 2008. 
So currently you need an already up-to-date FreeBSD CURRENT machine to install a FreeBSD with zfsroot on another disk.

Before building an up-to-date FreeBSD kernel and world, you have to set LOADER_ZFS_SUPPORT=yes in make.conf

After that its time for the step-by-step run-down. Please read the run-down first, try to understand what you are doing and then do it step-by-step or use the appended example script.

# Create a GPT label on your disc
gpart create -s gpt ad4

# Add a FreeBSD boot-partition which will hold the ZFS bootcode
% gpart add -b 34 -s 128 -t freebsd-boot ad4

# Too see how much space is left on your disc use
% gpart show ad4
=>       34  586114637  ad4  GPT  (29G)
         34        128    1  freebsd-boot  (64K)
        162   60313308       - free -  (29G)

# As you can see there are 29GB free space left. 
# Add a FreeBSD zfs-partition which will be the provider of your zpool
% gpart add -b 162 -s 60313308 -t freebsd-zfs ad4

# Add the GPT bootcode to the MBR and add the ZFS bootcode to the freebsd-boot partition
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 ad4

# To make sure the BIOS loads the MBR of the disc use fdisk to activate the first slice
% fdisk -a /dev/ad4

# Now we can create the zpool on the freebsd-zfs partition
% zpool create mypool ad4p2

# Your pool should now be mounted under /mypool
# Create the filesystem used for holding your freebsd-system
% zfs create -p mypool/ROOT/freebsd

# Now we create some we also would like to have in seperate filesystems
% zfs create -o mounted=no,mountpoint=/usr/local mypool/usr-local
% zfs create -o mounted=no,mountpoint=/tmp mypool/tmp
% zfs create mypool/var

# Now its time to install FreeBSD
% cd /usr/src
% make -s installkernel DESTDIR=/mypool/ROOT/freebsd/
% make -s installworld DESTDIR=/mypool/ROOT/freebsd/
% mergemaster -i -D /mypool/ROOT/freebsd/

# We need to fix /var so it is mounted correct when booting from ZFS
% zfs umount mypool/var
% zfs set mountpoint=/var mypool/var

# Enable the new filesystem as zpool bootfs
% zpool set bootfs=mypool/ROOT/freebsd mypool

# We still need to tell the kernel from where to mount its root-filesystem
# Add zfs_load="YES" to /mypool/ROOT/freebsd/boot/loader.conf
# Add vfs.root.mountfrom="zfs:mypool/ROOT/freebsd" to /mypool/ROOT/freebsd/boot/loader.conf

# Copy the zpool.cache to the new filesystem
% cp /boot/zfs/zpool.cache /mypool/ROOT/freebsd/boot/zfs/zpool.cache

# Thats it, you should now be able to boot into ZFS-root is you set your bios to boot from ad4


################ EXAMPLE SCRIPT ###################

#!/bin/sh

pool=$1
geom=$2
swapsize=$3

if [ "$pool" = "" ] || [ "$geom" = "" ]; then
        echo 'Usage <pool> <geom> (swapsize)'
        exit
fi

swapsize=`echo "${swapsize}"|tr GMKBWX gmkbwx|sed -Ees:g:km:g -es:m:kk:g -es:k:"*2b":g -es:b:"*128w":g -es:w:"*4 ":g -e"s:(^|[^0-9])0x:\1\0X:g" -ey:x:"*":|bc |sed "s:\.[0-9]*$::g"`
swapsize=`echo "${swapsize}/512" |bc`

gpart create -s gpt $geom
gpart add -b 34 -s 128 -t freebsd-boot $geom
gpart bootcode -b /boot/pmbr -p /boot/gptzfsboot -i 1 $geom

if [ "$swapsize" ]; then
  offset=`gpart show $geom | grep '\- free \-' | awk '{print $1}'`
  gpart add -b $offset -s $swapsize -t freebsd-swap $geom
  partnum=`gpart show $geom | grep 'freebsd\-swap' |awk '{print $3}'`
  glabel create swap /dev/${geom}p${partnum}
fi

offset=`gpart show $geom | grep '\- free \-' | awk '{print $1}'`
size=`gpart show $geom | grep '\- free \-' | awk '{print $2}'`
gpart add -b $offset -s $size -t freebsd-zfs $geom
partnum=`gpart show $geom | grep 'freebsd\-zfs' |awk '{print $3}'`

# Make first partition active so the BIOS boots from it
echo 'a 1' | fdisk -f - $geom

# Create the pool and the rootfs
zpool create $pool ${geom}p${partnum}
zfs create -o compression=lzjb -p $pool/ROOT/$pool

# Now we create some stuff we also would like to have in seperate filesystems
for filesystem in var usr-local tmp; do
   echo "Creating $pool/$filesystem"
   zfs create $pool/$filesystem
   zfs umount $pool/$filesystem
   _filesystem=`echo $filesystem | sed s:-:\/:g`
   zfs set mountpoint=/${_filesystem} $pool/${filesystem}
done

zfs set mountpoint=/$pool/ROOT/$pool/var $pool/var
zfs mount $pool/var

echo ####################################
echo 'Now install world, kernel etc'
cd /usr/src
make -s installkernel DESTDIR=/$pool/ROOT/$pool/
make -s installworld DESTDIR=/$pool/ROOT/$pool/
mergemaster -i -D /$pool/ROOT/$pool/

chmod 1777 /$pool/ROOT/$pool/tmp
chmod 1777 /$pool/ROOT/$pool/var/tmp

# We need to fix /var so it is mounted correct when booting from ZFS
zfs umount $pool/var
zfs set mountpoint=/var $pool/var

# Enable the new filesystem as zpool bootfs
zpool set bootfs=$pool/ROOT/$pool $pool

# We still need to tell the kernel from where to mount its root-filesystem
echo 'zfs_load="YES"' >> /$pool/ROOT/$pool/boot/loader.conf
echo "vfs.root.mountfrom=\"zfs:$pool/ROOT/$pool\"" >> /$pool/ROOT/$pool/boot/loader.conf
echo 'zfs_enable="YES"' >> /$pool/ROOT/$pool/etc/rc.conf

if [ "$swapsize" ]; then
  echo 'geom_label_load="YES"' >> /$pool/ROOT/$pool/boot/loader.conf
  echo "/dev/label/swap none swap sw 0 0" > /$pool/ROOT/$pool/etc/fstab
fi

# Copy the zpool.cache to the new filesystem
cp /boot/zfs/zpool.cache /$pool/ROOT/$pool/boot/zfs/zpool.cache