According to Wired Italy, Twitter (if you asking to yourself what Twitter is, you do need to have a look at it!) has suspended the planned and announced maintenance of the system pushed by the flood of requests.
In fact Twitter has been defined one of the few communication media available in Iran during this tough period.
Internet is having a real crucial role in this protests no matter the effort to control / close it, see what The Guardian writes:
Although many sites including Twitter, Flickr and Facebook are blocked in Iran, Iranians are using proxy servers to route around government-controlled gateways and publish images, video and blog posts about the protests.