Do you mean by the statement "at boot time", the message displayed when there is a successful login to a shell? (via telnet or at the console)
This is stored in /etc/motd (and can be edited with vi or pico)
Alternately messages can be added on a per user basis, by adding commands to .bash_profile file in a users home directory. So to issue a warning message;
echo "John your directory is very large could you please delete some of these files"
E-smith has done a lot of work getting this distribution together so they really deserve some announcement on login. However it is common on the machines that I log onto, for a legal warning to be displayed such as "Unauthorized use of this machine is illegal an may be punished by ritual stoning"!