Sending a message to all workstations

How do I send one message that pops up on the screen of each terminal in our lab? I'm thinking something like a popup window.

You can try wall, but I don't know if that creates a popup in GUIs or only works in CLI

See man wall

You could use a GUI program ( simply search the net ) and
then use SSH to execute it on each machine.

Here is a start: http://en.wikibooks.org/wiki/List_of...world_programs

Why a GUI? You could also use an Xterm window to do a simular function.

xterm -hold -T "Message from God" -e "echo -e '\t\t\tHello\n\tclose window by hitting the X'"

May other options such as changing colors for greater effects.

Log into each workstation using ssh
The message should be in the file message.txt
Execute this:
xmessage -file message.txt -center -display :0.0

Of course if you have a lot of workstations, this should be automated.

if you use xmessage then you don't need to use ssh, just use the display options.
you will need to set xhost to accept the command on each target machine.