We've seen a few powers that make a creature insubstantial now, but we haven't been able to find out what it means. What does it mean and where can we get a clear definition?
In 2nd edition you became 'insubstantial' because of a spell like ectoplasmic form, which turned you incorporeal or 'not real' in a way. So the effect in 4e (like Ectoplasmic Armor for example), similiar, in that you take half damage. In earlier editions you couldn't be hurt except by +1 or magical weapons, or by spells, when incorporeal.