On the x86 platform, a new GRUB keyword, $ZFS-BOOTFS has been introduced. When booting an x86 based system, if the root file system that corresponds with the GRUB menu entry is a ZFS dataset, the GRUB menu entry contains the -B option with the $ZFS-BOOTFS token, by default. If you install a release that supports a ZFS boot loader, the GRUB menu.lst file is updated with this information automatically. The default bootable dataset is identified by the bootfs property.
On x86 based systems that are running a release that supports a ZFS boot loader, this information is included in the GRUB menu.lst file.
For step-by-step instructions on booting a system from ZFS, see x86: Booting From a Specified ZFS Root File System on an x86 Based System.