Linux
Disable automatic startup of X on boot
Change the default run level in /etc/inittab to 3 id:3:initdefault:
Enable automatic startup of X on boot
Change the default run level in /etc/inittab to 5 id:5:initdefault:
Solaris 10
Disable automatic startup of X on boot # /usr/dt/bin/dtconfig -d
Enable automatic startup of X on boot # /usr/dt/bin/dtconfig -e