What is your iptables policy for the INPUT, FORWARD, AND OUTPUT chains?
Anything set to the target "DROP" in your FORWARD chain?
If not, could it be that your ISP blocks port 80 for some reason.
I'd try setting up anything (http server, telnet, ssh, etc) running on port 80 on your linux box (router, with port-forwarding not enabled), and try to access it from the outside. Then if that work successfully, but the portforwarding doesn't, you can be sure the problem has to do with the port forwarding.
If still doesn't work, please post results of the following commands:
iptables -n -L
iptables -n -t nat -L