Any vehicle that moves at all means no assault, Wether it moves at cruising or combat speed. The only exception is for Open Topped vehicles, which Droppods are, and thusly why this thread came about. It's only the "deepstrikers cannot assault" clause that keeps them in line.That and the fact that the drop pod is considered to have moved at cruising speed. Any vehicle that has moved at cruising speed, no assault for you. Only if it has a special rule allowing you to assault, like the Vanguard Vets with Heroic Intervention, IIRC.
Hope that helps a bit