Using latest trunk code, but bb_head works fine in combo with add_action here.
is_forum() doesn’t take any arguments, so I believe you’d have to use the global var $forum, like so:
<?php
/*
Plugin Name: noindex stuff
*/
add_action('bb_head', 'my_add_noindex_tag');
function my_add_noindex_tag() {
global $forum;
if($forum->forum_id == 3) {
echo '<meta name="robots" content="noindex" />';
}
else {
echo '<!-- please do index -->';
}
}
?>
Aha, glad I spent an extra couple of minutes searching, as I was about to ask this same question. bbpress looks great btw, I really like the slimline approach. 
