how do you enable the screensaver...
 

to run at the login prompt on eith KDM or GDM...

thank you.

AFAIK, can't be done directly in KDE. You configure the KDE login manager (KDM--K Display Manager) in the Control Center->System Administration->Login Manager. You can enable a slide show to somewhat help prevent screen burn-in.

However, some quick surfing reveals that you might be able to manually hack a screen saver option. Try starting here.

No idea, really, but I wonder if xscreensaver (as opposed to kde's screensaver) might do the trick.