Re: single forum
Search engines have no problems with 301 redirects.
http://www.tamingthebeast.net/articles3/spiders-301-redirect.htm
You could save this as your index.php and not have any trouble:
<?php
header("Status: 301 Moved Permanently", false, 301);
header("Location: http://www.yourdomain.com/forum.php");
exit();
?>
What are the reasons you don’t like redirects?