I understand that if you take the land route you should be OK, but if you fly you need a PR travel permit, check your nearest Canadian visa issuing office for details
Commercial carriers may not board you on the flight if they know you are a Canadian PR, but do not have the card. But if you are flying from the US to Canada, they probably won't care. Showing your US passport does not guarantee you entry to Canada. You have to undergo an examination and they will ask you why you are coming to Canada. It will be awfully strange if you do not inform them you are a PR at that time. I am assuming your visa is still in your passport, so they will know by looking at it. Anyway, not having the card does not strip you of PR status. If you can demonstrate you have PR status, either through the visa in the passport, record of landing or whatever, then your should be fine. It will just go a lot smoother if you have and show the PR card on arrival. If you do fly, you must also show your passport, while if you drive across a PR card should be good enough, but do not try driving back into the US w/o your passport.
