The previous report on this topic was closed by MadDogX after stating "You can autorotate by holding the "collective lower" key." which simply does not work (or at least has not yet after numerous trials).
While its obvious that it doesn't work like it should in a real life scenario, the descent isn't as fast as a falling helicopter without rotor movement would be (though no helicopter in Arma descends particularly fast while remaining level)
Video on how autorotation looks in practice - http://www.youtube.com/watch?annotation_id=annotation_757822&feature=iv&src_vid=Je1gI4-OLx8&v=h5WoDFwikfQ