Cut and paste the next two lines to add general login support
(and to allow root to log in over the console)

echo S0:2345:respawn:/sbin/agetty ttyS0 9600 vt102 >> /etc/inittab
echo ttyS0 >> /etc/securetty

In /boot/grub/grub.conf add the next two lines (near the top)

serial --unit=0 --speed=9600
terminal --timeout=10 serial console

Add:

console=tty0 console=ttyS0,9600

to the end of every kernel line.

(example kernel lines next. Note you must do this when upgrading
your kernels as well! You should check this after running 'up2date'!)

example:

kernel /vmlinuz-2.4.20-8smp ro root=LABEL=/ console=tty0 console=ttyS0,9600
kernel /vmlinuz-2.4.20-8 ro root=LABEL=/ console=tty0 console=ttyS0,9600

Comment out or delete the splashimage line (very important!)

#splashimage=(hd0,0)/grub/splash.xpm.gz

Example grub.conf:

#boot=/dev/hda
serial --unit=0 --speed=9600
terminal --timeout=10 serial console
default=0
timeout=10
#splashimage=(hd0,0)/grub/splash.xpm.gz
title Red Hat Linux (2.4.20-8smp)
root (hd0,0)
kernel /vmlinuz-2.4.20-8smp ro root=LABEL=/ console=tty0 console=ttyS0,9600
initrd /initrd-2.4.20-8smp.img
title Red Hat Linux-up (2.4.20-8)
root (hd0,0)
kernel /vmlinuz-2.4.20-8 ro root=LABEL=/ console=tty0 console=ttyS0,9600
initrd /initrd-2.4.20-8.img