I thought I remembered that when I got NEXUS I was told that a passport is required for air travel. But http://www.cbp.gov/travel/us-citizen...ocs-air-travel says "All U.S. citizens and nonimmigrant aliens from Canada, Bermuda, and Mexico departing from or entering the United States from within the Western Hemisphere at air ports of entry are required to present a valid passport, NEXUS card (if utilizing a NEXUS kiosk when departing from a designated Canadian airport), Merchant Mariner Document (for U.S. citizens traveling on official business), or Military Identification Card (for members of the U.S. armed forces traveling on official orders). Note that children are also required to present their own passport when traveling by air." So it might be that the requirements have changed.

In fact, http://www.cbp.gov/travel/us-citizen...vel-initiative says "What types of documents are accepted for entry into the United States via air?

U.S. citizens can present a valid: U.S. Passport; Trusted Traveler Program card (NEXUS, SENTRI or FAST);" so it appears that they have.