I personally think we (Canada) shouldn't participate in Star Wars. For one, if the Americans can't even run the Patriot right i don't trust missles from space coming down at us (if they should miss like the Patriot)
There is no urgent threat of NK using nuclear weapons, i'd be more afraid of the States using one (they hold the record for using them on other countries for the purpose of death and destruction)
It would be used as an offensive weapon. Let's be honest, you always have a hidden agenda (Iraq and it's oil for instance) and I don't think it would be used for only denensive purposes.
Money waster. The cost of this program is very high, there's not even the correct technology yet. I'd rather spend money on health care and other social programs than a war weapon.
It would most certainly spark another Arms Race. First weapons in space would want other countires to try and get their own programs for weaponized space...which wouldn't be good because then they would be used for an offensive weapon.
Plus, anything Bush wants to go forward with can't be good for the rest of the world.