For all you Canadian gardners out there - if you're looking at "zone maps" from the US, remember that they use a different system than in Canada. Typically, a Zone 4 plant from a US source will be a Zone 5 plant in Canada. A Zone 5 plant in the US will be a Zone 6 plant in Canada.
On top of that, a single province (like Ontario) can have up to 5 different zones in the province! I included more here, if you are interested: http://red-is-green.livejournal.com/9474.html