It's a good idea to add a password to the GRUB option for “recovery mode”. Without a password on this option, any knowledgeable person could log into recovery mode and change your passwords or do whatever they want.

To create this password as an md5 hash:

$ grub 

grub> md5crypt

Password: *********
Encrypted: $1$sEOFN1$dalP1fzlFCHD4saxlnNzM/

grub> quit

Copy this encrypted password and paste it in your grub config,

sudo gedit /boot/grub/menu.lst

add the password to the recovery mode entry like this:

title		Ubuntu 8.04, kernel 2.6.24-16-generic (recovery mode)
root		(hd0,5)
kernel		/boot/vmlinuz-2.6.24-16-generic root=UUID=********* ro single
initrd		/boot/initrd.img-2.6.24-16-generic
password --md5	$1$sEOFN1$dalP1fzlFCHD4saxlnNzM/

References: http://www.linux.com/articles/53569