to my mind, the best evidence for the mutability of IQ, i.e. for the importance of the whole environment on it, is the Flynn effect. It's obviously impossible for genetic change to account for anything like the magnitude of that effect, so those increases have to be environmentally caused, for the broadest possible definition of "environmentally".
I don't think anyone would argue that there are not environmental influences on IQ when you consider the impact that lead exposure or malnutrition, for example, have on brain development, and it seems clear that the gains that we have made in many countries in improving maternal and infant health and in decreasing toxic exposure to materials like lead (by banning its use in paints and fuels, for example) have had a beneficial effect on average IQ.
It is important to note that, because of the nature of standard scoring, decreases in the number of people scoring at very low levels shift the center of the curve to the right - which means that the population level IQ goes up. It is also important to understand that children with high "genetic potential" are not immune to the deleterious effects of poor nutrition and toxic exposures: an environmental insult that depresses IQ by 30 points leaves what would have been a gifted child functioning at an average level, and decreasing the incidence of such insults increases the proportion of the population able to function at a high level, again raising the average IQ. The Flynn effect over the course of the 20th century is a paean to the relatively steady progress that we have made world-wide in improving living standards and nutrition, improving pre-natal care, and decreasing pollution and childhood exposure to known neurotoxins.
There is also no question that deprived environments, those where children have few opportunities to explore or interact with their environments in novel ways, or where they have little exposure to language, depress IQ. But that is a far cry from indicating that IQ is infinitely elastic, as some (not you) have argued.
