Rendering with the default renderer
Formats supporting alpha channels: .tif, .tga, .png.
The Rhino 3 default renderer does not generate alpha channels [Pascal - 2005-11-09] but you can make one manually by saving a copy of the scene. Then change all the materials to pure white and the background to pure black. Turn the ambient light all the way to pure white as well. Delete all other lights.
As long as you don't change your animation or other rendering settings, this sequence should be a perfect match for your normally rendered animation. You can now use this matte animation as a mask in after effects or premiere or whatever compositor you are using to lay the animation in over your digital photograph. [MALaFon - 2005-11-10]
The built-in renderer in V4 beta does support alpha channels. Select File→Properties→Rhino Render→Transparent background before rendering, and then save the image in .tga, .png or .tif format. [Mikko - 2005-11-10]