Isn't this the plot of Idiocracy...stupid people have more kids?
Person A has an IQ of 102 Person B has an IQ of 104, therefore; the child will have an IQ of 103. There is no specific IQ gene and so I would imagine that it would take just as long from an evolutionary standpoint to decrease the average IQ as it would for us to loose our tails and stop living in tree's. It certainly wouldn't happen in a matter of 30 years. At least, I don't think it would... I could be wrong but I don't believe I am.
Birth control is currently the primary natural selection criteria. Anything that can defeat birth control is extremely strongly favored by evolution. Evolution is quite rapid when the selection effects are strong, especially when also starting from a diverse population.