DCUI security banner is defined in /etc/vmware/welcome. Run the following command to make it effect right away.
/sbin/auto-backup.sh
kill $(ps | grep dcui | awk '{print $1}')
SSH security banner is defined in /etc/issue. The change takes effect immediately.
Reference: http://www.virtuallyghetto.com/2010/09/how-to-add-splash-of-color-to-esxi-dcui.html
Jackie Chen's IT Workshop 