Since well before the Olympics actually. I was in China when 9/11 happened and for a brief period of time they opened access to a large number of previously blocked foreign news sites so that people could read what was happening.
As for how I discovered they were unblocked, I got a phone call from my Mum early in the morning after the attack (the attack happened during the Chinese night time) and went in to the office (which had the only available internet access), and one of my colleagues told me that the bbc and nytimes were no longer blocked.
Not sure how he found out they were unblocked, but he'd been there most of the night.
Then once you know one is unblocked you try them all :-)