I flew NW476 from ATL-DTW on May 24th. No problems. Later in the day, I checked the reservation on nwa.com and the E-Ticket Details page showed that the flight had been cancelled and offered me the opportunity to rebook. It still shows cancelled this morning (May 25th).

Obviously a glitch. It will be interesting to see if I my WorldPerks account gets credited for a flight that didn't fly!