Regardless of what screen saver module I choose, when the screen saver timeout expires, it doesn’t launch the screen saver module and it doesn’t blank the screen. If I wiggle the mouse, a login dialog pops up, but the screen is still not blanked.

This is opensuse 12.1, kde 4.7.2, nvidia 304.43.

I personally never bother with screensavers, they are more trouble than I care to think about.
But it should work…
Unfortunately I now use 12.2 and kde 4.9.2

What happens if you use the Test feature in the screensaver settings?