[Mediaaffect]

With Flash Player 8 compatible Flash video, you can use the On2 VP6 codec’s alpha channel feature to add a video mask to your content. You need to create the alpha channel in another video editing application, such as Adobe After Effects or Adobe Premiere. Once you have a QuickTime file (MOV) with an alpha channel, you can process the source video file with a Flash 8 video encoding utility to create the video transparency in the final FLV file.

…from another site I keep on file.