Dynamically create GUI in trigger?

How can I draw a gui box with text dynamically when a trigger is entered, then have it disappear when any key is pressed?

Use a boolean flag to activate the message

e.g.

var showMessage : boolean;
var someRect : Rect;
var someMessage : String;

function OnGUI{
   if(showMessage){
      GUI.Label(someRect,someMessage);
   }
}

To activate the message just set showMessage using a trigger

e.g.

function OnTriggerEnter(){
   showMessage = true;
}

To hide the message use:

http://unity3d.com/support/documentation/ScriptReference/Input-anyKeyDown.html

e.g.

if(Input.anyKeyDown){
   showMessage = false;
}