I know that crossbows have the same classification as compound and recurve bows in BC, and hence yes they are legal for hunting, both in bow and rifle season.

As for the rest of Canada, you would have to check all provincial regulations. Forthermore, I would venture to guess that you should also check into each provincial regulation book thoroughly, because I know in BC each region is governed by a seperate wildlife biologist, whom each sets different seasons for different types of firearms and different seasons for each species. Therefore it could be possible for instance to have one region legal to use crossbows in BC while another region in BC they are not allowed.

Hope that helps a bit - do a search of yahoo for "BC Hunting Regulations"or whichever province you are interested in, I know the majority of them are available electronically over the net.