The FAQ on this issue says "FlightAware presents the time that the aircraft departed and arrived, also known as the “wheels up” and “wheels down” times. At large airports or in various circumstances, there can be a significant difference in these times due to delays on the ground. "
There’s something rotten in Denmark here, because Air France says that flight 332 today (2011.1.1) departed CDG at 14:13 and Flightaware says wheels up was “13:45 CET”. It turns out that the Paris Airport says wheels up was 14:28!!