DDT is, in some respects, a lot like certain antibiotics, excepting that it bioaccumulates, ends up thinning the egg shells of birds, and has been linked, in chronic and/or acute exposure, to various maladies in people. (Remembering that it bioaccumulates, many people of a certain age in the west have detectable quantities in the fatty tissues of their bodies).
When it was first introduced, it was a miracle chemical--able to wipe out NEARLY all mosquitoes, and able to nearly single-handedly wipe out malaria in some regions (from the Wikipedia article: 3 million annual cases of malaria in Sri Lanka to just 29):
http://en.wikipedia.org/wiki/Ddt#Overall_effectiveness_of_DDT_against_malaria
But just like antibiotics they got overused, or used superfluously (come on, therapeutic doses of antibiotics, even on livestock, and antibiotic soap / lotion / tissues?) Plus that whole bioaccumulation thing (i.e. it builds up in animals, and throughout the ecosystem). Its strategic use (i.e. mass sprayings, supermarket availability, etc.) was responsible for destroying mosquito populations, and drastic disease reduction, and it was everywhere (in varying amounts). Unfortunately, DDT's great success was also it's biggest problem (and I think this dramatic success is also the reason why so many people are still enamoured of it).
Since mosquitoes usually lay several hundred eggs per pair, those few survivors quickly gave rise to DDT-resistant mosquito populations! WE (or maybe it was WHO), in effect, selected for DDT-resistant mosquitoes! And DDT's effectiveness dropped like a rock. In places (e.g. India) where they've continued strategic use of DDT, it's become completely ineffective (I hesitate to link to the Wikipedia article on such an important point, but the article does provide a link):
http://en.wikipedia.org/wiki/DDT#Mosquito_resistance_to_DDT
In order to keep on making use of DDT (in spite of its human effects, which are probably not as bad as getting malaria), it has to be used tactically--infrequent indoor spraying, impregnation in mosquito netting and bed clothing (despite being resistant, mosquitoes apparently will still stay away from it). Which is probably slower, more costly, and less dramatic (at least short-term) than, say, spraying it from planes and trucks. And I sure hope that its tactical use is what will result with this reversal (along with commensurate funding).
You might say WHO is scum for opposing DDT, but you can also vilify them for advocating (and using DDT) and causing DDT-resistant mosquitoes! (Who do you think launched those mass-DDT efforts in the 50s and 60s?) All-around anti-WHO weapon. Unless, like DDT, you use these too often, and people see you're using both sides of the same coin and develop a resistance to it. (This
http://www.colbertnation.com/" moment brought to you by DEET)
EDIT: Strategic and tactical in the above post is applied in the same way as to nuclear weapons:
Strategic: 200 megaton "Tsar Bomba" fusion bomb
Tactical: 0.5 kiloton "Davey Crockett" nuclear artillery shell