I want to start X app on display:0 remotely. How?

[kitepilot at kitepilot.com]

Hello X: 

This is the scenario: 

I have a machine running remotely which I can ONLY SSH into.
I know that the machine is running X and "operator" is logged in and can run 
graphical applications.
I'll use kcalc for the example. 

What I want is to: 

ssh -l operator remote.host
(insert some magic here that I know exists but I can't find and run) kcalc 

WHAM!
Now kcalc pops on display:0 and operator can see and use kcalc as if he/her 
had invoked it him/herself.
Mud?    :)
Enrique