Since the disputed elections in Iran, the Iranian government has blocked access to lots of websites and media. Protesters are currently using Twitter to communicate and inform the world about what's going on in Iran. Twitter doesn't want to risk a downtime during the riots, so it decided to perform the maintenance later today when it is night in Iran.
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.