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 you are worried then the best thing to do is call your vet or poison control center immediately.
For general reference 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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