Was in the F/D of BAW B757 at AMS many moons ago when we took off without ATC clearance. Informed by ATC after take-off. Embarrassed apologies to the tower to which he replied, "I was monitoring you and would soon have stopped you if I would have had to". Slightly different scenario since we were of course on the correct frequency. I would assume that in this particular case the TWR would have been monitoring the arrival and, failing to have contact, ensured a clear runway.