As the Iranian government tightens its grip on international reporters and social networking, one question still lingers: How is news still filtering out of the country by Twitter?

Time’s Lev Grossman has an explanation for how the service has managed to overcome the government’s crackdown, as well as manipulation by supporters of Mahmoud Ahmadinejad.

He writes, “Twitter isn’t a magic bullet against dictators. As tempting as it is to think of the service as a purely anarchic weapon of the masses, too distributed to be stoppable, it is theoretically feasible for a government to shut it down, according to James Cowie, CTO of Renesys, a company that collects data on the status of the Internet in real time. While Iran has a rich and diverse Internet culture, data traffic into and out of Iran passes through a very small number of channels. It’s technically relatively trivial for the state to take control of those choke points and block IP addresses delivering tweets through them. The SMS network is even more centralized and structured than the Internet, and hence even easier to censor.

“But there are counter-countermeasures to this kind of censorship. Sympathetic observers outside Iran have set up “proxies,” servers that relay Twitter content into Iran through network addresses that haven’t been blocked yet. When the Iranian authorities discover such a proxy, they block it too. It’s an arms race crossed with whack-a-mole. Protesters are also organizing denial-of-service attacks against government websites — coordinated efforts to shut down their servers by flooding them with traffic.”

Meanwhile, although the foreign press ban was in force today, one Middle East correspondent is defying it. I suspect we’ll hear more stories of this type of courage among reporters in the country in the coming days. BBC managed extensive video images of today’s protests. And there is still a stream of images filtering out from citizen journalists, like the footage uploaded to YouTube, below.

Dan Rather writes on the Daily Beast that the situation in Tehran “is not Tiananmen 1989.” The “proliferation of information technology and the phenomenon of citizen journalism have made it much harder now to turn the lights out than it was two decades ago. Oral history once kept alive for generations the stories unsanctioned by official propaganda; now social-networking tools have the power to spread the people’s story around the world, instantly.”

Update: James Longley, who directed the Oscar-nominated documentary “Iraq in Fragments,” has been in iran for some time shooting a new project. He was detained and released on Sunday after interviewing a woman about the protests on the streets of Tehran. Longley was part of a delegation organized by the Academy of Motion Picture Arts & Sciences that visited the country in February.