By commenting the following segment from dokuwiki\inc\auth\mantis.class.php, I was able to make single sign-on work even when dokuwiki is protected by apache auth:

 // Has a user name been provided?
        if ( !empty ( $user ) )
        {
            // User name provided, so login via form in progress...
            // Are the specified user name and password valid?
            if ( auth_attempt_login ( $user, $pass, $sticky ) )
            {
                // Credential accepted...
                $_SERVER['REMOTE_USER'] = $user; // Set the user name (makes things work...)
                $ValidUser = true; // Report success.
            }
            else
            {
                // Invalid credentials
                        if ( !$silent )
                {
                    sg ( $lang [ 'badlogin' ], -1 );
                }

                $ValidUser = false;
            }
        }
        else
        {

EDIT (dregad): markdown

I believe that call to 'sg' is a typo. It should be 'msg'.

I had to fix it to get PHP errors to stop when I set up my DokuWiki recently.

I've updated the preferred wiki page that most of the obvious links direct you to on the Mantis wiki.