QT with alpha channel

I´m editing a scene with chroma key, an artist standing talking, in FCP and would like to export the sequence to QT with the alpha channel separated - so I´ve heard it should be? The goal is Flash with different backgrounds, for internet.
Any suggestions someone how to perform this export properly?
I´ve read that the sequence not should be rendered in FCP before export, otherwise I get a black background. I now got a white background in QT, but the material still doesn´t work well in Flash.
/PonthoFilm