Just FYI, This is what a friend who works @ Port Hurnon US immigration told me:
Nexus is a United States Government Issued Identification. It is valid at ALL US entry points. If they do not have a specified "Nexus" line, you can still use the Nexus pass as your ID for entry into the US.
For person's entering other countries, including Canada, it is the "perogitive" of the custom's agent to request additional ID such as your passport to gain entry. It is advised to also carry your passport if you have it.
Nexus is currently good for travel to US, Canada, Mexico and Carribean....NOT CUBA.
From Canada Customs agent Sarnia: If you carry a valid nexus card you should have no problems using it to re-enter Canada.
We have Nexus, but also carry our passports ... even for day trips. WE LOVE NEXUS!!