1. Check opacity in the Motion canvas by turning on the Transparency view (checkerboard pattern will appear where BG is transparent).
2. Export the current frame as a Still Image and select the Photoshop file format.
3. Open the exported file in Photoshop. The background will be the same color as the Canvas color in Motion, but you will find an Alpha Channel in the Channels palette.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com