The following methods should help you attain a transparent background in Flash...
Just render the image in an image format that supports alpha channels (tga e.g.). Open the image in Photoshop and there should be a mask in the channels window. This is the alpha channel.
Another method that works for image sequences / animations:
First of all, make sure you have Quicktime installed. In Maya, render using tiff as your image format. In Flash, choose import and locate your image(s). Flash should ask whether it should import through Quicktime - do that... voila.
Hope it helps.