I flew AA from YYZ (terminal 3) to DFW on aug 13. Departure time 5pm. I can confirm that NEXUS entry to security was closed and just regular entrance to security was open. It was the regular process (shoes off, laptops out) to go through security. On positive note, the place was empty and was through in a few minutes! More screening staff than passengers. This may be due to terminal 3 and low volume of flying from airlines who use this terminal (AA, Delta, Westjet). I suspect this will change as more flights are added to US. And I have heard terminal 1 (T1) is much busier, so hopefully someone can share their T1 experience.

Once past security, NEXUS kiosks are operating to go through US customs. But I used main line (no one in line) and not NEXUS. The place was empty and I had my choice of 4 different custom officers to use.