Well, first off they have to have a special diplomatic clearance, since their criminal record is available to the canadians on the interpol system.

When they enter canada, instead of going through the normal customs process they have to go into a special room and talk to a supervisor...I've never been in the special room, so I don't know what they talk about.

If it has been over five years since your conviction and / or termination of your sentence, you may apply for a pardon through the consulate. (Probation does not count as a sentence) If it has been ten years, then no pardon is necessary as they find that you have been rehabilitated enough. If it has been under five years, don't bother to apply, as no pardon will be considered. The DUI in Canada is in fact a felony charge, so they take it much more seriously then us in the states. I am not sure what the actual process is at the border but I think that you have to provide the proof of pardon from the magistrate when asked about your background.

oh, and it costs a few hundred dollars, payable to Canada for the pardon.

A few hundred $CN...atleast it's cheap.