No need to be a lawyer, Washington residents have already been told not to try and bring back any BC bud across the border. EDIT: because one is entering the U.S. where it is illegal, not WA where it is legal. Once past the US border, one is then in the weed-friendly state of WA. /EDIT Conversely, our neighbors to the north have made it a point to remind WA residents that weed is still illegal in Canada. Source: stuff I tripped across while looking up border crossing procedure for a recent trip, which I could probably find again if I had to.