Catnip affects the pheromone receptors in domestic cats and some wild species of cats (Cheetahs and catnip are a trip). Only like 2/3 to 3/4 of the cat species are susceptable to the effects of catnip. If I remmber right, cats from Australia generally aren't because they have a relatively closed genetic pool.
It is harmless as far as we know as long as the stoned cat doesn't roll down the steps into the basement (it happened to my neighbors cat when I lived in Ohio).