The PS3 won't have vibrating controllers because the plastic they're made of is cheap and flimsy, says Immersion, maker of vibrating controllers. These sound like harsh words until you consider that they guy who said them, Immersion CEO Victor Viegas, has successfully sued Sony for infringing on their vibrating controller patent and now stands to lose quite a bit of licensing cash.
Sony, however, still blames the non-vibrating design on the gesture-based hardware inside the controller which allows you to move the controller around a bit in three dimensions. Sure, Sony. Sure.