Country-by-country doesn't make sense, either. What happens (in the future) when you hit a country like China 11% of the population wants to upgrade all at once (that would be about 150 million people)? Your plan FAILs.By releasing the software updates country by country (perfectly possible with the different iTunes stores) instead of waiting until US sales started to release them for everyone.
It isn't rocket science.
No rollout plan would ever work for the internet griefers. There would be comparisons to Microsoft, Bill Gates, the devil, and class action lawsuits because it's all become such a sport to lambast Apple.
Oh, and from the instant karma file: Both of the iPhones I was upgrading made it through the activation process while you were busy complaining. Enjoy your wait.