I completely agree with Sam here. I think the fallacy in the above argument is that there is a qualitative difference between Turing-complete machines and special-purpose machines. Turing-complete mechanization is broad and endlessly adaptable.
Programmable machines aren't machines. They're machine-machines, and can be adapted to new tasks in short linear time by small numbers of people with little capital. That makes it "different this time."
I also think that malicious AI is already kind of here, but in a hybrid "cyborg" form. It's the corporation. Corporations that destroy human livelihoods and abuse human beings in general to maximize per-quarter shareholder returns are a bit like "paperclip maximizers."
http://wiki.lesswrong.com/wiki/Paperclip_maximizer
The danger is not in some Terminator-like AI apocalypse, but that incremental advances in AI will make these things progressively less and less human and more and more machine. I can imagine a future almost-entirely-silicon financial corporation that uses its speed and superior analytical intellect (at least in the financial domain) to lay waste to entire national economies in order to maximize shareholder value... i.e. paperclips. Since this would likely be found in the hedge fund world, nearly all of this siphoned-off wealth would be captured by a small number of already very rich people.
Nightmare AI wouldn't be much like Skynet -- a new being pursuing its own self-interest. It would be more like a very, very smart dog helping its elite owners "hunt" the rest of us in the financial sphere. This could fuel even more massive consolidation of financial wealth. We are already seeing the beginning of this with algorithmic quant finance.
In a thread on Twitter I also heard someone bring up "AI assisted demagoguery," a notion I found to be total nightmare fuel. Imagine a Hitler wannabe with a massive text-comprehending propaganda-churning apparatus able to leverage the massive data sets available via things like the Twitter and Facebook feeds to engage in high-resolution persuasion of millions and millions of people. The thing that makes this scary is that populist demagoguery gets more appealing when you have things like massive wealth inequality.
You can make counter-arguments here, but I also agree with Sam that it is foolish to just hand-wave these kinds of possibilities away. We should be thinking about them, and about how -- as he puts it -- we can find ways to channel this trend in more positive directions.