The last time I rented a car in SEA I was asked if I was taking it into Canada. When I said yes, they asked me if I had ever been convicted of DUI (no). It appears that a DUI conviction makes it illegal for anyone to cross the border into Canada...
Edited to say I did more research. There -many- criminal convictions that will keep you out of Canada...
http://gocanada.about.com/library/weekly/aa111900a.htm
[This message has been edited by birdstrike (edited 08-22-2003).]