There's a good discussion going on in the comments section on BMD so I thought I'd elaborate a bit more on the topic. Paul Wells got a great cross-section of comments on the topic last month - feel free to check them out here.
This is an issue I can see both sides of. I can respect the "it's retarded, but who cares?" position. Basically, if it doesn't cost us a cent, why not go on board to help US relations, right? Mind you, I don't think this will have a huge impact in US relations. People keep pointing to softwood and mad cow, but I suspect that these trade tariffs are in place to please interests in the US. I can't imagine tariffs suddenly going up because of Canada's non-decision on Star Wars. You also have to look to Iraq. What if the US agreed to pay to send 200 Canadian troops there? It wouldn't cost us a cent. Yet many would still be against it if it's perceived as "wrong".
Is this as bad as Iraq? Well, no. But Martin has said that his government is against the "weaponization of space" and that's the road we're heading down. There have been numerous quotes from people in the Pentagon that the long-term goal of missile defense is to weaponize space. I know some people say we'll have a "seat at the table" but I can't imagine the US listening to Canada on this in a million years.
Even without the weaponization of space, it's a terrible idea. Every test done to date has failed miserably. It's like shooting down one bullet with another bullet. And it's costing billions. And for what? What nation in their right mind would send a missile to the United States, knowing they'd get obliterated 20 minutes later. No state is going to take the US on. The real threat comes from terrorist groups who will smuggle dirty bombs into the country or sail ships full of explosives into harbours.
All this program serves is a way to appease the neo-cons and weapons programs. It's an utterly stupid idea. If the US wants to do it, then fine. But I can't see any benefits to Canada joining.