You almost had it, if they can get your server page then your router is passing them through just like it is supposed to. I think the problem is the link you are providing to people, for a link try http://serverort/listen.pls
if they want to manually type it into Winamp the server
ort should be enough. It sounds like the link you provided is just server
ort or people are typing server
ort into there web browser.
Give that a try and if you are still having problems post back here and we'll see what we can do (also if you post back put up your server
ort so we can see what page you are talking about (it is just the shoutcast server page where you can see the last songs you played, right?))