Thanks for playing, everyone! Loree/danger garden is the winner. Coincidentally, she just had a poisonous plant scare and had posted on it yesterday (linkie given). There were lots of other great stories, however, and I must say I am feeling very, very uneasy about what I may be allowing to infiltrate my neighbors' gardens.
The wicked plants most often mentioned? Poison ivy, bindweed, black locust, and ailanthus.