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
996374bb5a