I did a quick test on my 2003 SBS server.

I forwarded port 80 to the IP of our server through the firewall, and
was able to get this.

You are not authorized to view this page
(Apparently, IIS requires authorization of subnets or specific
addresses to access the remote web workplace)

Open IIS Help, which is accessible in IIS Manager (inetmgr), and search
for topics titled About Security, Limiting Access by IP Address, IP
Address Access Restrictions, and About Custom Error Messages.

I'm personally reluctant to allow any IIS server open access to the
net.
I can use the remote web workplace successfully by connecting to my SBS
server using PPtP (or any other remote user vpn protocol).
Once I have a valid IP for the office network, I can browse to
http://192.168.18.3 (server IP) in any browser, and run the remote web
workplace.

To setup PPtP (a weak but easy to configure vpn protocol) go to
routing and remote access, and you have to have a firewall capable of
forwarding the protocol, not just the port to the static IP of your
server. I know previously, no Linksys routers where capable of
forwarding the protocol.
Dlink, Sonicwall, Watchguard fireboxes, and Cisco Pix are all capable
of forwarding the protocol. Beyond that, they all offer appliances
that allow with license, remote user vpn configuration directly to the
firewall.

Hope this helps some.