Comment 29 for bug 339902

hmpf - works for me:
  while :; do kdialog --passivepopup "This is a test" 5; sleep 10; done
then lock the screen.
intel gfx, no compositing (unlikely that it matters).

it's logical that the notification stays on the screen once it gets on top of the screen saver - for performance, many savers redraw only the actually changed contents. and as they don't expect strange things to happen, they don't react to regional repaint requests, either.

somebody who can reproduce the problem please paste the messages from krunner_lock (4.1) / kscreenlocker (4.2) found at/near the end of ~/.xsession-errors (provided kdm is used, otherwise you have to figure out yourself) after unlocking the screen.