From the Twitter blog:
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).
Update: Xeni sez, "Twitter didn't reschedule the downtime just because they were mensches, we now know they did so at the request of the State Department."