Chretien would have knocked Bush over the head with his brass balls.
That said, I think that it would probably be a good idea for Canada to get onboard with the BMD in a limited way, because the US is going to go ahead with it regardless of our participation. It would just suck to find out one day that we have missiles pointed over our territory and not have any say in when or where they get shot at.
The US needs us for NORAD, as well. Without the DEW line, BMD is useless.