The only real, true piece of identification that says that you are, beyond a shadow of a doubt, a Canadian Citizen is a passport. Period. Yes they can be faked, but it's harder to fake a passport than a birth certificate and driver's license.
Any country has a right to refuse entry to any person for whatever reason they feel like. For all intents and purposes when you're dealing with an immigration officer, you're dealing with "the country". If they decide you're not going in, you're not going in. Period. If they decide that your ID looks faked, you're not going in.
A passport isn't going to guarantee anyone entry to the US, but it does lessen the chances of being turned away. The official word from the Goverment is that, strictly speaking, a passport isn't *necessary*, but is highly recommended for travel outside of Canada.
And come 2008, a passport will be necessary for travel to the US anyway.