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.
Why can we assume that all the little things affect intelligence independently? Are synergies obviously rare, and how rare do they have to be for the central limit theorem to apply? In the simplest alternative model I can think of, incremental advances could be multiplicative instead of additive, which gives a log-normal distribution instead of a bell curve. This case is uninteresting because you could just say you're measuring e^intelligence instead of intelligence, but I can imagine more complicated cases.
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?