[CentOS] Message display

Is there a command in CentOS4 or some small program to display a short 
message everywhere possible on a box much like # shutdown -k now 
"Display me where I'll be seen", but without the added (normal) stuff?
xmessage would probably be usable if it had an -onalldesktops option.

BTW, the purpose is to add a visual to the scripts I have in place to 
remind me to connect my external USB drive late Tuesday, disconnect it 
early Wednesday (after backup) and keep it out of the reach of "Murphy" 
for the rest of the week.