Here's a quote from the Canadian Owners and Pilots Organization's Guide to Cross Border Flight Operations:


Licences and Certificates
Licences that meet ICAO standards, including all Private Pilot, Commercial Pilot and Airline Transport Pilot Licences (Canada) and Certificates (US) are valid to fly your aircraft on either side of the border.
Other permits and licences are not necessarily valid on the other side of the border. For example the Canadian Pilot Permit – Recreational Aeroplanes is only valid when flying a Canadian ultralight in the US under the blanket authority document. The Pilot Permit – Ultra-light Aeroplanes is not valid for flying in the US, unless you are flying a Canadian Ultralight and have an instructor rating and two hours of cross country time, as required in the blanket authority. Likewise US Recreational Pilot Certificates and Sport Pilot Certificates are not valid in Canada. Student Pilot Permits are not valid outside the issuing country.