Henry Farrell isn't convinced:

[W]hile Twitter (like SMS) can be used to organize protests on the fly, I haven’t yet seen any evidence that it made a substantial difference to organizing efforts in Iran. This is not to say that it didn’t - but we need good evidence (which will require Persian language expertise, obviously) of correlation between specific bursts of Twitter communication and forms of social protest etc before we can really be sure that there was an effect. What we can say is that previous instances of ‘color revolution’ relied much less on technology than you would have thought from reading Western media. New technologies tend to be less reliable and more easily disrupted than traditional forms of organizing - while they are surely becoming more important over time, I think it is fair to discount some of the more breathlessly enthusiastic reporting until the actual evidence comes in.

But in disseminating information, it has been critical. Do your bit to prevent Twitter going down for maintenance tonight. The email is support@twitter.com. Be polite.