Detecting lateral speed could be misleading for the device. As other posters have said, many cars will accelerate / decelerate faster than an aircraft.
1. A threshold speed could be used. Cars sold in Europe are limited to 155mph (unless a geek has plugged his laptop into it and tweaked things) Therefore if the device detected a speed higher than this (either by gsm or gps) it could switch off.
2. Alternatively the system could detect a sustained rate of climb. Even a 152 can climb at 500 ft / min whereas there are very few roads which would facilitate this. GPS could detect this with sufficient accuracy (yes, gps can detect altitude. Not accurate enough to fly by, but enough for this). If the climb continued for more than a minute (i.e. takeoff), the device would turn off.
3. The two systems above could be combined with a database of airport locations, so it could only be triggered when the device is at (or near) an airport. This would eliminate spurious activations of flight mode.
Hope that helps.