Actually yes, there are contract rules floating around. Chrysler had
them for the PT Cruiser when the CDN price was less that the US and
the US dealers were upset that CDN cars came into the US.   There are
also differences in some equpment.   You only think that the US has
stringent emission rules until you come to the CDN border and find
that in Canada the rules change.  This is kind of an odd thing,
particularly if you buy the car in Canada, register it at home, then
drive it anywhere in north america.  All of a sudden it's OK if the
emissions or other equipment is not to the other countries standard -
kind of a grandfather clause.

Then there is the issue of the speedometer.  The US models will have
the miles in 'bold' and the Kms in 'lowercase'.   You will constantly
be looking at your speed in miles especially in the dark.   In Canada
and the rest of the world it's the other way around.

You cannot just hop across the border, buy the car and drive it home.
You should immediately copy the purchase agreement or 'title', and
MUST send the originals to the US Port of Entry attn Duty Supervisor.
They will want this paperwork for at least 72 hrs before the car
arrives or the car will sit in their lot until they complete the
checking they do.  The copies stay with the car incase you run afoul
of the law - expect to do a lot of explaining.   US Port of Entry will
give you paper after the car is OK to proceed.  You WILL need this
paper for Canada Customs.

How will you get the car home?  US transit stickers may not be valid
in Canada and does your DMV allow you to take an existing plate with
insurance and place it on the car??  Alway have Insurance on the car.

If you proceed;  DOCUMENT eveything.  Phone calls made when, what was
said and to who - get names.  Copy all original paperwork and keep it
separate from the originals.  

Call Canada Customs and ask about importing a car - what can and can't
be done, what paperwork is required, what is excluded.

Call your US port of Entry.  This is the border station that you WILL
use to cross into Canada.  You want their mailing (courier delivery)
address and exactly who to ATTN the paperwork to.  

If the courier people say they can deliver the documents to the US
port of Entry the next day, they're lying especially if you buy the
car on Friday or if you pick an obscure entry point.  

The car I bought in California was almost 20 yrs old.  The rules
change for new cars.

Don't cheap out on phone calls, they're cheaper that the alternative.

Pete in Winnipeg