Cool, I think that's exactly it. Until now, I had overlooked that feature in appstoreconncet. Thanks a million!

OK, thanks a lot for your comments. How I would identify them is easy: if they write me an email, and I think they are one of the beta testers, then I'd give them the code for free download :-)

Thanks a lot for your response! I have just checked with a pristine account: screensavers quit when I press any key, just as expected. So, I am more puzzled now: what could be causing this odd behavior with my account? PS: my macbook is connected to one external display, with the lid closed.