It used to be a simple hack, scaling lens with 100, and moving the camera to the back with an equivalent amount. Because of the hack, making it 100% compatible with older files I could not achieve (yet?). To help reminding users, I've added a print when reading old files with Ortho cameras. Full description of how it works can be found here; http://www.blender3d.com/cms/Render_changes.515.0.html