Right now, the rules are staying the same (primarily Golden Age books) for the sake of consistency while we all get our footing. The current date was also partly chosen for convenience of research, not necessarily an absolute definition.
However, behind the scenes, we've been kicking around what will eventually be permissible and the standards on which to judge that while making it clear what's permissible and how thorough the research was (because, especially with a scan, you can't simply trust a single bad notice.
I've been looking at various candidates looking for bad copyrights as late as 1988, so may be some interesting stuff coming, once we're all feeling free to move in that direction.