UPDATE: Google in Iran seems to be back up in the past day.
The Iranian government is blocking access to https:// services on the internet, meaning any encrypted site is unavailable to Iranian users. This includes Facebook and Gmail.
It's not clear why the sites have been blocked -- in fact Iran's official Information Technology Organization told Iran's state news agency it is unaware of any problems.
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Speculation is the blockage is related to the anniversary of the Iranian revolution.
Iran may be working on its own internet-like system to try to satisfy its population, with a special "clean" search engine called "Ya Haq". Iran's head of communication, Reza Taqipour Anvari announced it would launch in early 2012
The US meanwhile is ready with its own "shadow internet" to bring connectivity to citizens in police states.