It's too easy to call the weekend's activities the first revolution that was Twittered, but when histories of the Iranian election are written, Twitter will doubtless be cast as a protagonal technology that enabled the powerless to survive a brutal crackdown and information blackout by the ruling authorities.
Had the revolution not been twitted, we'd still know about the misaligned election results, and given the hard work of traditional journalists -- ABC's Jim Scuitto, the New York Times's Bill Keller, a legion of correspondents for European newspapers -- the West would have some idea of the counter-Ahmadinejad protests.
The Twitter technology added two elements to this. Number one -- as Iranian authorities shut down internet servers, it allowed younger protesters, particularly those affiliated with universities in Tehran, to organize and to follow updates by Mir Hossein Mousavi; by spreading the word about the location of government crackdowns and the threat of machine-gun-wielding soldiers, it probably saved the lives of any number of would-be revolutionaries. We don't know how many Iranians belong to Twitter; there seems to have been about two dozen active voices from Tehran, but if we assume a multiplier effect -- these 24 people can coordinate with their 20 friends -- the use of the technology as a central organizing hub that circumvented official channels of communication cannot be understated. In this way, Twitter served as an intelligence service for the Iranian opposition. There are even hints that, once Iranian authorities figured this out, they attempted to spread misinformation via Twitter.