Well, central limit theorem says it's mostly a bell curve among humans
Only subject to some major assumptions.
Not that major. The assumptions are that there are many small, independent things that affect intelligence. These assumptions are wrong, in that there are many things that do not have a small effect at all. But to the extent that these (mostly bad things) are rare, you'll just see a bell curve with slightly larger tails.
Recently I stumbled upon Richard Carrier's essay "Are We Doomed" (June 5, 2009), when asked to comment about the Singularity, said the following:
What do you think?