Ottawa,
The rules vary in accordance to the type of aircraft that the departure aircraft was, but in all instances the departure must be airborne prior to the arrival crossing the landing thresshold. In the case that you describe, maybe the controller issued a go around but the pilot elected to land anyways, or the controller felt that it was far safer to let the RJ land than to risk an over/under situation, or from the tower it appeared that the B737 was airborne. The perspective between being on the ground or up in the tower is entirely different, so how do you say who was right or wrong?
Whether the rule is different in Canada I don't know. I can say that in my years of experience I've seen some go arounds issued by controllers that became quite scary because they ended up with an over/under situation between the go around and the departure. In these situations, in accordance to the rules the controller did the right thing, but as a controller you have to put SAFETY FIRST, and sometimes it would had been far safer to had let the arrival land than to had issued a go around.
I'll vote for SAFETY FIRST!
Mike