Adapted from http://gentoo-wiki.com/HOWTO_Gentoo_Install_on_Software_RAID :
If you are using a RAID 1 mirror disk system, you will want to install grub on all the disks in the system, so that when one disk fails, you are still able to boot. The find command above will list the disks, e.g.
grub> find /boot/grub/stage1
Now, if your disks are /dev/sda and /dev/sdb, and /boot is its own partition at /dev/sd(a-?)1 , do the following:
device (hd0) /dev/sda
This will install grub into the /dev/sda MBR, and
device (hd0) /dev/sdb
...will install grub onto the /dev/sdb MBR. The device command tells grub to assume the drive is (hd0), i.e. the first disk in the system, when it is not necessarily the case. If your first disk fails, however, your second disk will then be the first disk in the system, and so the MBR will be correct.
...repeat for all physical disks that have a mirror of /boot.
The grub.conf does change from the normal install. The difference is in the specified root drive, it is now a RAID drive and no longer a physical drive.