Render Smoke on Alpha (Transparent) Background?

It should be so easy! (But its not!)

Silly me - it turns out the domain object forces you to render the sky.

How exactly do you render smoke on a perfectly transparent background without any hackish / sloppy methods?

Eh? No it doesn’t. Make sure z-transparency is on for the domain material and your alpha mode in the render tab is set to “transparent” not “sky”.