We haven’t yet used this ability, but if and when we are required to withhold a Tweet in a specific country, we will attempt to let the user know, and we will clearly mark when the content has been withheld. As part of that transparency, we’ve expanded our partnership with Chilling Effects to share this new page, http://chillingeffects.org/twitter, which makes it easier to find notices related to Twitter.
It's an interesting approach.
eb: “…if we receive a valid and properly scoped request from an authorized entity, it may be necessary to reactively withhold access to certain content in a particular country from time to time.”
Twitter is not placing an automated censorship system in place, but rather will only comply with what it sees are valid requests.