It's relatively common knowledge that the Chinese government blocks certain content in China. Most of this won't affect Chinese users. But does iChat censor messages with said content in them?
This Yahoo Answers question says that the content won't be blocked unless it's pornographic or politically extremist. However, I'm not sure if Apple lets China access their messages, so I'm not sure if the Chinese government could do this.
Unfortunately, I don't have a way to test this myself. I've done some research, and I haven't been able to find a definitive answer, thanks to the fact that I can't read Chinese.
Example: I send a message to person 1 in China saying 'Down with the government! Tank Man! Tibet! Liu Xiaobo! Tiananmen Square protests of 1989!' Does Person 1 get the message or not?