Originally Posted by TRAVELSIG
Good you asked on this one- Canada is usually a real stickler for a Canadian in a US registered car entering Canada- it is specifically NOT allowed. Now, they may not catch it, but if they do- they will advise you to turn around or leave the car in impound until you return to the US border. Interestingly, the other way around is not a problem. Be careful on this one...
I rented a car from SEA - YVR a long time ago. The car they gave me at SEA had British Columbia plates and I had no problems getting into BC. I've also had no problems doing this recently from YYZ-BUF one way. I have not tried to do a return but I have done one ways from BUF-YYZ as well with no problems (with Ontario plates)