Please don't make the beta exclusive for North America from start! Alot of companies do this and then roll the beta to the other continents ignoring the fact that most people don't mind playing with a bit higher ping if it's a great game. Part of the testing should also include high ping behaviour :p so please please pleeeease, if you've considered this, don't!
And remember: if the apocalypse is global, the beta should be too!
In gaming terms... Life's a die, then you bich.