I have recently worked on one project that included fire simulation and I used new Blender 2.71 cycles CPU rendering of volumetric fire + smoke. Everything went well using a https://www.youtube.com/watch?v=oSiV5gg_lCs as a tutorial.
Problem arises when I wanted to render the fire over the transparent background/film. Transparency was messed and fire+smoke where too opaque. I tried everything and didn’t solved the problem.
Today I replicated the case and still found not answer to my problem. I am sure it some bug since in the preview windows it looks good but when I render it actually out - its bad.
Here are some screenshots to give you idea what I am talking about.
On the left side you can see the render and on the right side window render preview. Notice the fire, its more opaque which is bad.
It looks good when I turn transparent film off.
Node setup for domain material.
I tried every option in node editor plus every option in render/world tab and problem is always same.