Iran has begun to block Tor traffic. Tor is a program that allows you to anonymize your Internet traffic by disguising where it comes from. By blocking Tor traffic Iran is basically telling its citizens that it will monitor their Internet traffic, and it will prevent them from disguising it.
This is the beginning of an arms race, essentially. People who want to avoid censorship will turn to other means, encrypted means of passing traffic along. New services have already cropped up that allow users to disguise their data on the Iranian Internet, and everywhere that oppressive governments rule the day.