In the viewport, smoke looks fine:
However, when I render it with the OpenGL render:
It looks white because of the background; it's really transparent.
What causes this?
I would consider that a bug.
It appears that when the smoke flow object is set to Fire or Fire+Smoke it renders transparent with opengl render.
The fire simulator uses it's own material settings to preview in the 3dview - not the material settings that we configure. It would appear that this preview material works in the 3dview but not the opengl render.