Secure GRUB

Version 1 (aka 0.98)
Reference cyberciti.biz

1. #grub-md5-crypt
Password:
Retype password:
$1$NYoR71$Sgv6pxQ6LG4GXpfihIJyL0

2. # vi /boot/grub/menu.lst (Red Hat /boot/grub/grub.conf)
   Edit file and add a password line as follows:
   password --md5 $1$NYoR71$Sgv6pxQ6LG4GXpfihIJyL0

Sample config file:
default 0
timeout 5
password --md5 $1$NYoR71$Sgv6pxQ6LG4GXpfihIJyL0
title Debian GNU/Linux, kernel 2.6.13.4-cust-en-smp
root (hd0,0)
kernel /boot/vmlinuz root=/dev/hda3 ro
savedefault
boot

Version 2 (aka 1.98)
Reference ubuntuforums.org