[ILUG-BOM] Wall command like utility/command in Linux

I have 25 thin clients with builtin mozilla browser. I have a LAMP applicaiton running on a server. The users are accessing the application on their thin client using the builtin mozilla browser.

My problem is :

1) when i want to shutdown the server , i didn't know whether somebody is using the application or not.

2) I have another server on which the users are logged in through X session called on their thin clients. If some/any users forgots to log-off his/her login then I didn't come to know that the users is not doing anythink.

Is there any wall like command/utility , thru. which i can pass on a message which will be displayed on all the active users as well as the message will be passed to all the http users ?

-Jagdish