This came up on another thread and I thought I'd put it out there:
From the FAA clarification:
"With regard to cancelled flights, if a flight is cancelled before takeoff, then it does not count as a segment for Table B purposes. This is because a flight segment consists of a takeoff and a landing, and the lack of a takeoff/ landing means that there is no flight segment. However, the taxi out time for the cancelled flight segment would still constitute FDP time because the taxi out would have taken place after the flightcrew member reported for duty with the intention of conducting a flight. 30 If the aircraft moved under its own power for the taxi out, then the taxi out would also count as flight time because the aircraft would have moved under its own power for purposes of flight. "
ALPA'S FTDT guide dated May 2013 says that it IS counted as flight time, since the airplane taxied under its own power with the intention of flight, but the next revision from November says it is not because there was no landing.