I went through at T1 YYZ this AM and got an X on my Global Entry form (it is not giving me my new L1 visa on the entry options yet), and spoke with the officer. I asked if you need a passport, and he said that you absolutely need to have your passport in addition to Nexus. The US Customs can be inconsistent (thus why I have an L1 now), but I don't think it would be worth it to leave the passport at home. I have actually forgotten mine and travelled successfully, but not something I would