Answer: In general it is a good idea to assume they are poisonous unless you know otherwise as many of the most common ornamentals (such as yew) are in fact poisonous. Most are also (I hear) extremely nasty tasting so the likelihood that any one would eat them in any quantity is pretty low. If someone has eaten a certain berry and you are worried then the best thing to do is call your poison control center immediately.
For general reference however you might find the following two web sites about poisonous plants interesting: Poisonous plants in Canada (many grow here as well) http://res.agr.ca/brd/poisonpl/ and http://www.ansci.cornell.edu/plants/plants.html from Cornell University.
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