From the BBC:
Prof Stephen Hawking, one of Britain's pre-eminent scientists, has said that efforts to create thinking machines pose a threat to our very existence.Story.
He told the BBC:"The development of full artificial intelligence could spell the end of the human race."
Hawking is just the latest thinker to give this warning, which is articulated in great detail by Nick Bostrom in the book Superintellgence (which I happen to be reading; highly recommended).
Until recently, the dangers of AI have been relegated to science fiction -- Hal in 2001 is still the prime example. Now we see them in thrillers (I've done it in Dreamland: Collateral Damage, for example); next it will be real life - or maybe, if some of these predictions are correct, we won't see it at all.