The iTunes Store in a country is intended for use only by that country's residents, and only while they are in the country. To use the iTunes Store in a country you need a credit card (or other card type if acceptable in a country) issued in that country, billed to an address in that country, and also be physically present in that country when using the store. You are also restricted to waiting 90 days between country changes.
http://www.apple.com/legal/internet-services/itunes/us/terms.html#SERVICE - "Our Services are available for your use in your country of residence (“Home Country”)."... " Any other use of the Services and Content is a material breach of this Agreement. Apple may monitor your use of the Services and Content to ensure that you are following these Usage Rules."
The old wording was more specific: "The iTunes Service is available to you only in the United States, its territories, and possessions. You agree not to use or attempt to use the iTunes Service from outside these locations. Apple may use technologies to verify your compliance."