From Handling url binding failures in IIS Express:
Serving External Traffic
To enable your website to serve external traffic, you need to configure HTTP.sys and your computer's firewall. From an elevated command prompt, run the following command:
netsh http add urlacl url=http://myhostname:8080/ user=everyone
After configuring HTTP.sys, you can configure IIS Express to use port 8080 by using WebMatrix or Visual Studio 2010 SP1 Beta+, or by editing the applicationhost.config file to include the following binding in the sites element. (Replace myhostname with your computer's domain name).
<binding protocol="http" bindingInformation="*:8080:myhostname"/>
You will also need to configure the firewall to allow external traffic to flow through port 8080.