Recently I was asked about customizing the standard ESXi splash screen with a security message. While it is easy in ESX using a standard Linux approach, not as obvious for ESXi. However a quick google search found a nice little blog post on customizing the "welcome" file in /etc/vmware on the ESXi host.

Enable remote tech support mode and SSH to the ESXi host. Edit the /etc/vmware/welcome file and edit as needed. To see the work, log in to the ESXi server console and logoff to see the updated splash/login screen.

The file does support some very limited formatting options.

Colors: white,black,dark-grey,light-grey,yellow

Useful Macros: assettag,esxproduct,esxversion,hostname,ip,memory,servicetag

Here is an example text file:

{bgcolor:black}
{color:light-grey}
        {esxproduct} {esxversion}

                Hostname: {hostname}
                IP Address:  {ip}
{/color}
{align:center}{bgcolor:yellow}{color:black}This a secure/sensitive system.  Authorized access only.{/color}{/bgcolor}{/align}

Ref: http://www.virtuallyghetto.com/2010/09/how-to-add-splash-of-color-to-esxi-dcui.html