This subject came up on liketoride's Washington tour on Saturday.
Once you are clear of the border towns and need to get gas, you'll need to put a zip code into the pump when using your credit card.
Rick said he remembered a secret code that supposed works. A quick google located the below thread. I haven't read the entire thing, nor have I tried it yet. But it seems if you enter the 3 numbers of your postal code followed by two zeroes then your Canadian CC will work.
Ill be trying the next time I'm at a pump that requires it and will report back.