Originally Posted by
Albion
Depending which crossing you use the immigration agent may ask you to pull over and in some cases you have to go into the office. Can take 20 minutes or so.
Just what happened to us, except that we stayed in the car.
In our case, we were returning to the US so still had the I-94 and I-94W. We entered Idaho, where I guess they don't get too many US-registered cars driven by Brits

but no problem.
BTW, if you stay on the way and make certain purchases (in my case clothing and shoes but in principle anything non-consumable), don't forget to claim your Canadian GST (tax)
http://www.cra-arc.gc.ca/E/pbg/gf/gst176/README.html. There should be an agency just before you cross the border.