Now this is a very interesting concept. If you do business only in the US, this could be useful in further securing your network. If you run your own Exchange server, this could even tremendously lower your incoming spam volume at your network point of access (firewall). If you host on your own webserver, this could easily further secure it. Face it, the vast majority of security breaches and spam originate from outside of the US, and if you don't communicate with people or do business with people outside of the US, why allow any connections from those countries?
I give you the link: http://www.countryipblocks.net/continents/
I may investigate this further for our own use.