Yes Peace Lily's are poisonous! Ingesting the leaves in humans causes burning of the lips, tongue, and throat and is toxic if large amounts are eaten--it can also cause severe mouth pain. They are also poisonous to dogs and I was told can cause kidney failure but I don't know what amount would have to be ingested to cause this.
I was told by a botanist that water should be left to sit out for 24 hrs.and at room temp. before using to water a Peace Lily so the chlorine can evaporate--if not, the leaves will yellow and can lead to falling off and/or causing the tips of the leaves to turn brown. They also need an occasional misting to prevent brown tips.