On October 9, U.S. Customs and Border Protection (the CBP) released a Statement on Canada's Legalization of Marijuana and Crossing the Border (which can be found here). In it, the CBP advises:
A Canadian citizen working in or facilitating the proliferation of the legal marijuana industry in Canada, coming to the U.S. for reasons unrelated to the marijuana industry will generally be admissible to the U.S. However, if a traveler is found to be coming to the U.S. for reasons related to the marijuana industry, they may be deemed inadmissible.
Although this statement may provide some comfort to Canadians who are involved in the legal Canadian cannabis industry, it is important to remember that cannabis remains illegal in the U.S. at the federal level. The statement provides:
Generally, any arriving alien who is determined to be a drug abuser or addict, or who is convicted of, admits having committed, or admits committing, acts which constitute the essential elements of a violation of (or an attempt or conspiracy to violate) any law or regulation of a State, the United States, or a foreign country relating to a controlled substance, is inadmissible to the United States.