Although I agree with the idea of a rather inclusive definition for planet, including everything in orbit is going a bit overboard, because a classification should tell you something about the object so you know where to begin your thinking about it. And note that "planetary nebula" is a particularly unfortunate term to base an argument on-- it is a terrible misnomer, that comes not from the idea that the gas is still orbiting the star (it isn't), but rather that in a poor telescope such a nebula looks a bit like a planet!