I havent worked with IIS in any great amount of detail, however I know in Apache you can create Virtual Servers. Basically it creates two virtual IP addresses.
So if you wanted to run two websites, one at
www.somedomain.com and the other at
www.fakedomain.com, you would forward it all to the one IP of the webserver in your DNS. When the request arrives at the webserver, Apache will look at the destination and display the correct website.
I know you can do Virtual Directories in IIS..not sure about Virtual Servers though...although you could probably do it if you looked it up.