Gates was replying in a Reddit AMA (ask me anything)
http://www.reddit.com/r/IAmA/comments/2tzjp7/hi_reddit_im_bill_gates_and_im_back_for_my_third/
How much of an existential threat do you think machine superintelligence will be?
Gates, “I am in the camp that is concerned about super intelligence. First the machines will do a lot of jobs for us and not be super intelligent. That should be positive if we manage it well. A few decades after that though the intelligence is strong enough to be a concern. I agree with Elon Musk and some others on this and don’t understand why some people are not concerned.”
He also talks about creating an intelligent agent, “One project I am working on with Microsoft is the Personal Agent which will remember everything and help you go back and find things and help you pick what things to pay attention to. The idea that you have to find applications and pick them and they each are trying to tell you what is new is just not the efficient model - the agent will help solve this. It will work across all your devices.”