There has been no official reason for the block, although according to Reuters, “The Iranian Students' News Agency (ISNA) said Google ban was connected to the anti-Islamic film posted on the company's YouTube site which has caused outrage throughout the Muslim world.”
Another possible reason is to ease the Iranian public into accepting the planned official Iranian intranet, largely cut off from the world wide web. Iran is already one of the most heavily censored countries in the world, with some estimates suggesting that 5 million websites are being blocked. The national network would allow the government to switch from an unwieldy web blacklisting approach to a more effective whitelisting approach, allowing access only to approved sites and services. It would also make the use of foreign proxy sites and VPNs to defeat the censorship far more difficult.
According to Reuters the switch to the government network is already under way. “In recent days, all governmental agencies and offices... have been connected to the national information network," deputy communications and technology minister Ali Hakim-Javadi was quoted as saying by the Mehr news agency. The second phase of the plan would be to connect ordinary Iranians to the national network, he said.”
This national network is expected to be complete by March 2013. It is not yet clear, however, whether general access to the global internet will be severed at that time.