Okay, for those that want to hide the "Post un-anonomously" ticky box from all users except admins and mods, here's what you need to do:
Open /Themes/default/Post.template.php
// anonymous board mod
if ( $context['anonymous_board'] )
echo ' '. $txt['Post_anonymous'] .' <input type="checkbox" name="post_unanon" />';
// anonymous board mod
if ( $context['anonymous_board'] && $context['user']['is_admin'] || $context['anonymous_board'] && $context['user']['is_mod'] )
echo ' '. $txt['Post_anonymous'] .' <input type="checkbox" name="post_unanon" />';
If you chose to also display that line on your Quick Reply box then:
Open /Themes/default/Display.template.php.
Repeat as for Post.template.php.
Once I learn how to add it as an option under board settings, then I'll post the code here. For now you'll just have to hardcode the membergroups.