Read this:
www.cbp.gov/ImageCache/cgov/content/pub ... ngacar.doc (download openoffice or the word file viewer from Microsoft if you haven't got Word).
The bit it's vague on is that there is no duty on vehicles imported as personal effects.
Once you've got the full CBP clearance after the EPA test etc. some States may try and charge you sales tax when you go to swap the title at the DMV, the CBP clearance is supposed to exempt you from it so obviously getting that bit of paper is critical.
Bear in mind a Canadian-spec vehicle is a bit of an oddball in the US so it might not be worth it depending on what it is and what you can sell it for in Canada. Even if you change the fuses for the headlights, get the odometer and speedo changed, etc. (which isn't legally required) the VIN will always be Canadian and a dealer may not like that.
The vehicle does not have to be accompanied, there is a CBP form (Form 3299) for
shipping it separately, but obviously it's going to cost you more to do it that way and the freight forwarder will be talking to CBP rather than you, which I don't think is a good idea generally (as they can screw up and get duty charged on it).
Steve.