Well this is bad news. I had a hunch the stuff was evil, even though I've been slathering it on like skin cream when I travel to malaria-infested places. Some 200 million people a year use the stuff, and apparently it's bad news for the brain and nervous system, particularly when combined with other repellents that have similar toxic effects. Snip from a Science Daily article:
The active ingredient in many insect repellents, deet, has been found to be toxic to the central nervous system. Researchers say that more investigations are urgently needed to confirm or dismiss any potential neurotoxicity to humans, especially when deet-based repellents are used in combination with other neurotoxic insecticides.
Vincent Corbel from the Institut de Recherche pour le Développement in Montpellier, and Bruno Lapied from the University of Angers, France, led a team of researchers who investigated the mode of action and toxicity of deet (N,N-Diethyl-3-methylbenzamide). Corbel said, "We've found that deet is not simply a behavior-modifying chemical but also inhibits the activity of a key central nervous system enzyme, acetycholinesterase, in both insects and mammals".
Here's the source report: Deet inhibits cholinesterase: Evidence for inhibition of cholinesterases in insect and mammalian nervous systems by the insect repellent deet (BioMed Central)