the laws of robotics

I guess most science fiction readers would know of Isaac Asimov's 3 laws of robotics. How many of you realise that there is a fourth law? I didn't... till today. Apparently Isaac Asimov added a law zero to the other three:

0 - A robot may not injure humanity, or, through inaction, allow humanity to come to harm.
1 - A robot may not injure a human being or through inaction allow a human being to come to harm.
2 - A robot must obey the orders given it by human beings, except where such orders would conflict with the First Law.
3 - A robot must protect its own existence, as long as such protection does not conflict with the First or Second Laws.