I was told during my interview @ YVR that a verbal declaration at the border is sufficient nowadays.
I came back from Seattle on Sunday, and asked both the US & Canadian border guards at Peace Arch about the whole Nexus card/passport thing. The US guard said that all the lanes have the Nexus card reader, so you just drive up, flash your card, talk to guard, and you're good to go. No need to bring passport.
The Canadian side didn't have any of the Nexus card scanners in the non-Nexus lanes, so I gave the Canadian guard both my passport & Nexus. He just took the Nexus and gave me back my passport. He said that both are shared and legally recognized forms of photo identification, but it's still best that you have your passport on you.
Basically, we're back to have no idea which one is correct
