The Wall Street Journal has a free ed-op site. One of the editorials today (well, Friday), by Mary O'Grady, discusses the risk that Canada creates by failing to police its immigrants.
She brings up several incidents, including the Algerian terrorists who tried to cross the border with bomb-making chemicals (she incorrectly called them "plastic explosives") in late December of 1999. The man driving the car, whose nervousness at U.S. Customs got him caught, was a known terrorist, but Canada had failed to do ANYTHING about him, not even after he withdrew his "refugee status" request.
To quote the author, "Without a firm commitment from Canada to [implement and enforce counterterrorist policies], the only other option would be to close the border."
http://[url]http://www.opinionjournal.com/wsj/?id=95001129[/url]