Hi Tim, this is possible by enabling history updates on the mirrored object.

To do this, select the new mirrored object (the _output_ from the mirror command), and run Edit / History / Enable update.

After that, you can then turn on control points of the original and edit it, and the mirrored object should update automatically.

History updates are not enabled by default for all the Transform commands. I'm a little bit worried about automatically turning on history all over the place, since it can sometimes lead to unexpected results if you wanted to edit the newly created objects as independent objects. Maybe mirror should be an exception though.

- Michael