It looks like Canadians can register and participate in Open for Questions:
In fact, the site even accepted my Canadian postal code in the zip code field. I also did not see any terms of service, or check this box before proceeding prompts. Either way, it looks like a real interesting system. I wonder how long it will take until we see Canadian copy cats.
UPDATE: I doubt it will take long as the service is being powered by Google Moderator.