Australia
Austria
Belgium
Brazil
Canada
Denmark
Finland
France
Germany
Ireland
Italy
Mexico
Netherlands
New Zealand
Norway
Russia
Spain
Sweden
Switzerland
United Kingdom
United States
Anywhere else on the world? Well tough luck says MS but they are thinking of expanding it. And this is what MS said when they got questions about it in support:
Yep MS will not support every country in the launch. . . . . . . . . .
Source: http://metro.co.uk/2013/06/13/xbox-one-will-only-work-in-21-countries-at-launch-3839195/
http://www.thatnaijablog.com/2013/06/xbox-one-wont-work-in-all-countries.html