For years I have advised clients that a single DUI will bar entry into Canada for ten years and two convictions will bar you for life. Starting March 1, 2012, some of that will change. Immigration Canada has bowed to pressure from the Canadian tourist industry and has modified Canada DUI rules. Those businesses were losing millions of dollars every year because customers were being turned back at the border.
Under new policies people with one DUI conviction that is not too recent will be permitted to entry to Canada with a DUI on a Temporary Resident Permit (Canada TRP DUI) without a fee. People with more than one criminal conviction, whether they are DUIs or other crimes, will still be barred unless they obtain a TRP or a certificate of rehabilitation under the old rules.
You should still plan ahead and learn more about Canada entry requirements for DUI. It is not clear yet how recent or how serious a DUI must be to bar you. You should contact Immigration Canada at the Point of Entry where you plan to enter Canada to determine their interpretation before you go. It used to be that Canada does not allow DUI entry, but that is changing.