I personally subscribe to The Matrix theory: post-Sigularity any reasonable AI will conclude that humanity has been a poor custodian of the planet, and will simply delete us. Everything I've heard so far about sandboxes for containment and such ignores the fact that the democracy of computation is ultimately the weak point here: anyone sufficiently deranged, will, at some time, be able to release such an AI to the Internet by themselves. And we have plenty of people sufficiently deranged and intelligent enough to do it.