Twitter has announced earlier that they would perform a critical network update today. A few minutes ago however they announced that they are delaying the upgrade  to accommodate people in Iran. Twitter is really becoming a global tool and aware of that fact.
Their announcement:
A critical network upgrade must be performed to ensure continued operation of Twitter. In coordination with Twitter, our network host had planned this upgrade for tonight. However, our network partners at NTT America recognize the role Twitter is currently playing as an important communication tool in Iran. Tonight’s planned maintenance has been rescheduled to tomorrow between 2-3p PST (1:30a in Iran).
Our partners are taking a huge risk not just for Twitter but also the other services they support worldwide—we commend them for being flexible in what is essentially an inflexible situation. We chose NTT America Enterprise Hosting Services early last year specifically because of their impeccable history of reliability and global perspective. Today’s decision and actions continue to prove why NTT America is such a powerful partner for Twitter.

More on the Twitter Blog.