Here's an interesting article about blogs as a venue of free speech in Iran. It talks about how people have found the "anonymity" of blogs allows people to "take their masks off" and actually express themselves. (Interesting language--it's the kind of vocabulary you often find in dissident movements. It's also interesting in light of discussions about ethics and anonymity that have been going on here and elsewhere in the blogosphere.) It also notes that the conservatives in Iran have not yet implemented the laws that bring all ISPs under government control--but that they might do this.
The article is worth a look. One of the standard cases for a medium having a decisive impact on political events is the use of radios leading up to the Iranian Revolution. Iranians, in the past, have been able to use communication technology to support dramatic political change. It sounds like this might be a case where it could happen again--or it could be shut down.