In a slight change in policy, Twitter just announced that it enabled the ability "to reactively withhold content from users in a specific country — while keeping it available in the rest of the world". Basically, if a tweet is illegal in a certain country, it will be censored in that country. In the rest of the world, it'll still be completely visible.
Previously, Twitter had no control over tweets other than removing them globally (which they don't do). Twitter says it's introducing this policy because Twitter is growing internationally and into countries that have "different ideas about the contours of freedom of expression". If Twitter didn't change the policy to include the ability to censor, Twitter wouldn't be able to exist there.
Twitter hasn't censored any tweets yet, but it'll likely happen as new countries get a hold of the service. Once the tweet is censored, it'll be clearly marked with a link to an explanation. For what it's worth, Google and other search engines already participates in a similar policy for certain countries. It'll be interesting to see how this affects tweet-roots campaigns in other countries. [Twitter via MarketingLand]