Owning a pet, especially those kept indoors, means being diligent about what they're to eat...and what they're not supposed to eat. Our cats had the habit of chewing on indoor ferns and palm leaves until we sprayed them with lemon juice; we lost a few plants, but we'd rather lose them than our feline family members. We've recently added another household hazard to the list: the Sago Palm (not even a real palm, but a relative of the spruce and douglas fir) contains an extremely toxic seed that can cause seizures, coma and death in both humans and animals alike!
The ornamental plant's seeds contain cycasin, as well as a related compound called neocycasin, which both release cyanide upon ingestion. Best to keep the Sago Palm out of homes with either pets or small children. And here's a more definitive list of poisonous plants, provided by the Humane Society, worth perusing for safety's sake.
(Re-Edited from 2007-10-10 - CB)