Maybe its open in your server but closed in your provider's firewall (some providers do that to prevent hacks and worms), if that's the case you will only be able to connect to the sql port from inside your server.
By the way, I don't agree with mozartz, not every port is open... by default all of them are CLOSED, you need a program (daemon) that will sit and listen on a port to OPEN it, and therefore it can be BLOCKED by some firewall system (or the new port blocking feature of windows). Those rules apply to every known OS.
Every port IS open by default - i.e. the ports are not actively blocked whether there is anything listening on the port is another matter.
Regards the port 1433 are you sure it's closed ? - have you tried telnetting to it ?
If it IS closed and there's nothing on your or your providers network blocking it have a look in SQL server properties for which network protocols have been installed - it might be that tcp/ip was not included (though it is by default) or that it's set to listen on a different port.
If this server is open to the world you should think about moving it to a non-standard port anyway before the next slammer appears.
Ah, one more thing - how are you connecting to the server (?) - some ISP's also block 1433 so if it's a customer or something that can't connect that could be the reason.
1433 seems to be like a swear word to most network admins