The Address resource documentation under the People API describes the country_code attribute as "country code". However, it appears the standard being used is ISO-3166-1 alpha-2 with upper-case characters. This is a requirement, entering a code which doesn't comply with this causes problems with NationBuilder's front-end. However, these requirements are not documented nor are they verified when an address resource is submitted.
Great point Jeremy. The documentation has been updated to reflect this: