CGK says that it has been determined that they aren't actually poisonous if eaten. However, buying them has been partly responsible for over 20 million infants being slaughtered over the past 33 years.
I found this, since everyone seems to trust Snopes:
http://www.snopes.com/holidays/christmas/poinsettia.asp,
when trying to find another reference on "poisonous" poinsettias. My mom is a master gardner, so she was my original source.