rxvt: execute script when focussing special window

[baschni]

Hi there!
I've got a messenger, running in console (centerim) in X. I have written a tiny script so that I am notified, when a new message arrives; this will make an icon
on the taskbar blink. Now I want to stop this blinking when I switch back to my centerim-Window. Is this possible with rxvt or any other terminal emulator? I could identify the window by the process id of the application running or by the title or anything, when this window is focused I want to execute a script.
Thx in advance,
baschni