The myth that Poinsettia (Euphorbia pulcherrima) plants are toxic still persists. However, extensive testing has shown that eating Poinsettia causes, at worst, only minor physical discomfort.
Scientists at Ohio State University tested Poinsettia toxicity in rats (a standard model of toxicity testing). The rats were fed large amounts of various parts of the Poinsettia plant. None of the rats showed any signs of illness–no changes in behavior, appetite, body weight gain, etc.
Poison control centers keep records of suspected poisoning cases in humans. A review of those records found few cases of illness in children or adults who had eaten Poinsettia, with the worst symptoms being mild nausea and vomiting. In 1975, the U.S. Consumer Product Safety Commission, which oversees and monitors the potential toxicity of consumer products, reviewed the data and concluded that Poinsettia was non-toxic.
Nonetheless, the rumor persists, spread largely by word-of-mouth. People like to err on the side of safety, which is good. However, in the case of Poinsettia, this concern is unfounded. It is a wise idea to keep all houseplants away from rabbits, other pets, and small children. However, there is no reason to single out this beautiful plant.
An updated plant toxicity list can be found in the House Rabbit Handbook, 5th Edition.