I was asked this question this week, and wanted to share the answer.
Question: When I render particles with an Alpha Channel and import that into Flash, there is a dark halo or weird edges around the particles, which makes compositing impossible. I am rendering out as an FLV with Alpha. What am I doing wrong?
Answer: Render with Alpha channel, but as a straight Render. This should solve your problem. This is almost always the solution to problems like this. If you don’t know about Straight Renders, I have a short tutorial called “Straight Vs. Premultiplied” That you can watch here.
There’s also a follow up tutorial called “Remove Color Matting” which may be useful to you as well, when dealing with the halo issue.