Search the forum for "Getting into Canada DUI". You will find the following post. Apparently, Canada is looking beyond the 5 year "rehabilitation" period.
I had a buddy that this happened to. You can get spiecal VISA that allows entry, it'll take at least a year to get it. He had no idea that there was such a law, his DUI was 9 yrs old. Anyway he had entered the country as a crew member (pilot) maybe 30 times and never been questioned; one day he airlined in as a passenger, thats when they caught him. He got to enter, had to pay $250 for temp VISA, but once he left he couldn't reenter. Anything like this gets flagged when you go through immigration, which generally doesn't happen to a crew member. Customs has nothing to do with it.
The word he got during his stay was that Canadian Immigration Officers have a lot of leeway in enforcing this policy. The past 3 years they have become sticklers because of the the US's ridiculous policy of requiring passports when traveling between the 2 countries.
Note: If the DUI is over 10 years old, all is forgiven. Basically my buddy's 10 years was up before he ever got the VISA.