You can modify /etc/issue and /etc/motd as rockdw suggested, but if when you start you don't see the desired message, check your /etc/rc.d/rc.local
Some distro's put some code in here that will overwrite your /etc/issue every time you boot.
Mine looks like this:
# This will overwrite /etc/issue at every boot. So, make any changes you
# want to make to /etc/issue here or you will lose them when you reboot.
if [ -x /usr/bin/linux_logo ];then
/usr/bin/linux_logo -c -n -f | sed -e 's|\\|\\\\|g' > /etc/issue
echo "" >> /etc/issue
echo "$R" >> /etc/issue
echo "Kernel $(uname -r) on $a $SMP$(uname -m) / \l" >> /etc/issue
It's easier to comment out all this part (add # in front of every line) and your file won't be overwritten!