Most Canadians are rabid anti-Shrub. It would be political suicide for the minority Liberal party to be seen too closely involved in what is viewed by most as another GWB adventure that provides his associates with money but very little in the way of a practical defence.
I'm against anything that might result in a nuclear weapon being brought down over my head.
Not that I think any nuclear attack against the US would be missile based. Much easier to ship it over in a container of cheap TV's.