I would think the key line of attack in trying to describe why a singularity prediction is reasonable is in making clear what you are predicting and what you are not predicting.
Guys like Horgan hear a few sentences about the "singularity" and think humanoid robots, flying cars, phasers and force fields, that we'll be living in the star-trek universe.
Of course, as anyone with the Bayes-skilz of Eliezer knows, start making detailed predictions like that and you're sure to be wrong about most of it, even if the basic idea of a radically altered social structure and technology beyond our current imagination is highly probable. And that's the key: "beyond our current imagination". The specifics of what will happen aren't very predictable today. If they were, we'd already be in the singularity. The things that happen will seem strange and almost incomprehensible by today's standards, in the way that our world is strange and incomprehensible by the standards of the 19th century.
The last 200 years already are much like a singularity from the perspective of someone looking forward from 15th century europe and getting a vision of what happened between 1800 and 2000, even though the basic groundwork for that future was already being laid.
I appear today on Bloggingheads.tv, in "Science Saturday: Singularity Edition", speaking with John Horgan about the Singularity. I talked too much. This episode needed to be around two hours longer.
One question I fumbled at 62:30 was "What's the strongest opposition you've seen to Singularity ideas?" The basic problem is that nearly everyone who attacks the Singularity is either completely unacquainted with the existing thinking, or they attack Kurzweil, and in any case it's more a collection of disconnected broadsides (often mostly ad hominem) than a coherent criticism. There's no equivalent in Singularity studies of Richard Jones's critique of nanotechnology - which I don't agree with, but at least Jones has read Drexler. People who don't buy the Singularity don't put in the time and hard work to criticize it properly.
What I should have done, though, was interpreted the question more charitably as "What's the strongest opposition to strong AI or transhumanism?" in which case there's Sir Roger Penrose, Jaron Lanier, Leon Kass, and many others. None of these are good arguments - or I would have to accept them! - but at least they are painstakingly crafted arguments, and something like organized opposition.