I have tried your site a couple times and it works fine.
From your site I see posts, that you are aware that if a user is not logged on they will not see the shout box. I would assume it is a cookie browser/issue. Since I visit a lot of ubbsites sometimes when I logon it is not accepted using IE7. So I end up clearing my cookies the hard way on my desktop station.
To try to narrow it down change your permissions so that everyone can use the shoutbox. At the very least that would tell you is it a permissions issue or a browser issue.
