Foreign Policy has a fascinating article on the role of Twitter in the Iranian protests.
A number of opposition activists have told me they used text messages, email, and blog posts to publicize protest actions. However, good old-fashioned word of mouth was by far the most influential medium used to shape the postelection opposition activity. There is still a lively discussion happening on Facebook about how the activists spread information, but Twitter was definitely not a major communications tool for activists on the ground in Iran.