Search indexing

Well, today I was looking through google and I noticed none of my forum pages were getting indexed. I decided this was obvioulsy because I forced all users to login in order to view the pages. I know that it kinda defeats the purpose of forcing uses to login, but still have them be able to view the forums by viewing cached google pages. Anyways I threw together a quick hack

    $agent = $_SERVER['HTTP_USER_AGENT'];
    if (strstr(strtolower($agent), "mozilla") == false)
        $pun_user['g_read_board'] = 1;

for the default user function, and yes I know it sucks. Basicly I am wondering if anyone knows of a better way to only allow spiders to view the forums, but still disallow users