BillConner wrote on Feb 12^th, 2011 at 6:19pm:

"Non-Canadians - Requirements to Enter Canada. When you enter Canada, a CBSA officer may ask to see your passport and a valid visa, if one is necessary. If you are a citizen of the United States, you do not need a passport to enter Canada. However, you should carry proof of your citizenship, such as a birth certificate, certificate of citizenship or naturalization, as well as photo identification. If you are a permanent resident of Canada or the U.S, you should bring your Permanent Resident Card with you."

I've yet to meet a customs agent as agreeable as this.  I'm certainly not a gov't official or an expert so I can only speak upon 20 years of experience of going through Fort Frances nearly 10x per year, Rainy Lake (Baudette) 4-5 times per year, Pigeon River 2x per year, Pembina 1-2x per year, Lancaster 1x per year and misc. times through Windsor.  (My sales route and bagpiping takes me up there quite often).  However, I will concede as I cannot speak to southern Quetico entry since I have never done this since the law went into effect.

I will only say "good luck" getting back into the US w/o one though.