I too feel this pain acutely, to combat it I wrote a small and VERY basic JavaScript file a while ago to change this on the fly on a webpage based on your GeoIP location.
https://github.com/MartinMcGirk/English.JS
Canada was a toughie that I meant to get around to later. I was working from the reference at http://wikitravel.org/en/English_language_varieties which has three categories of English; "Commonwealth English", "US English" and "Canada".
It didn't really give me enough detail on Canadian English to create a new category there and then though so I left it as a "I should get around to looking this up soon".
My apologies to the fine people of Canada. I'll look at rectifying it shortly.
You know the way to our hearts. Personally, I think defaulting to Commonwealth is sufficient for Canadians. On a side note, does it pluralize "octopus" differently by region? I've always been a big fan of "octopodes" as the plural, which is not really accepted anyways.
US, unfortunately. I'm not convinced of the accuracy of the script's list of commonwealth countries. I could have sworn Canada was part of the commonwealth.
It is a very valuable member of the Commonwealth, but as I understand it it uses a mix of Commonwealth English and US English and I didn't get round to including it as an option.
Also, please can you add localisation? The word "donut" makes me feel physically sick. http://foaas.com/you/%22donuts%22/lotsofcows