A simple install on WordPress 3.4.1 will destroy the formatting of the "Add New Post." This problem is not hard to fix. I continue to use this plugin on WP 3.4.1, however, you must make the following change in order for it to work without destroying the formatting of "Add New Post":
Edit the plugin file "semiprivate_comments.php" or from the Plugins Editor (Plugins->Editor, then "Select Plugin to Edit": Semi-private comments).
Change the line in "function plugin_spc_sidebar()" from:
<div id="semi_private_post_dbx" class="dbx-box postbox">
That fixes the problem.