"Ever try pot? Answer yes to a border agent, and foreigners could face permanent consequences even if they haven't used marijuana in years. More and more Canadians are learning the hard way that admitting to U.S. border agents that you smoked pot can bar you from entering the country forever. Immigration lawyers say some Canadians are under the mistaken impression that legalization of marijuana in Washington state has resulted in leniency by U.S. border agents here, but it hasn't. Marijuana is still an illegal substance under federal law." - KPLU, Mar. 29, 2013.
[NOTE: Immigration attorneys know that the issue of 'admissions' is much more complicated and nuanced than described in the above article. See Matter of G-M-, 7 I&N Dec. 40 (BIA 1955), affirmed 7 I&N 40, 85 (A.G. 1956). For an example of how this plays out in practice, see this Mar. 7, 2005 AAO decision.]