as redsoxs says any soap or shampoo will remove oil from their feathers and skin. this will reduce the waterproofing of their feathers meaning they will stay wet longer in rain or snow - a definite problem when it is cold. they will preen themselves to re-oil their feathers. it will also change the condition (pH etc) of their skin which could make them more prone to skin infections etc. chickens don't need to be washed normally. if they managed to get themselves extra poopy warm water will clean it off. i would only use shampoo if they got something like motor oil on them.
i'm sure some people do wash their chickens, and a mild shampoo would be the best, without any harm coming to their birds but it really isn't necessary.