I haven't yet heard any critic of Singularity religion claim that strong AI is never possible.
What we are skeptical of is that we need just a little bit of development and strong AI will take-off just like that without any regard to physical world constraints.
Well, one critic I usually hear about singlularity subjects is that "you can't upload your mind into a computer because the emotional/soul/bla aspects of a human being could never be emulated"... which is obviously religious. Yes, it's very very hard but certainly no reason to call it impossible.
What we are skeptical of is that we need just a little bit of development and strong AI will take-off just like that without any regard to physical world constraints.