The main purpose for this is to allow rendering motion blurred blender fluids in external renderers (eg. http://vimeo.com/21870635 ). Python code snippet for interpreting this data here: http://www.pasteall.org/21577 . Cleaned up some ugly hacks in this area too * Also added read-only access to scene.subframe to RNA - setting current frame and subframe should still go via scene.frame_set()