I know this post is a little bit old, but for future reference.
http://www.aspca.org/site/FrameSet?style=User&url=../toxicplants/M01947.htm
This is what they say about the Peace Lily.
Common Name: Peace Lily (aka Mauna Loa Peace Lily)
Scientific Name: Spathiphyllum
Family: Araceae
Toxic Principle: calcium oxalate crystals
Clinical signs: oral irritation, intense burning and irritation of the mouth, lips, tongue, excessive drooling, vomiting, difficulty in swallowing.