According to the New York Times (http://www.nytimes.com/2010/01/13/world/asia/13beijing.html):

Google did not publicly link the Chinese government to the cyber attack, but people with knowledge of Google’s investigation said they had enough evidence to justify its actions.

So I think it's a matter of the Chinese government seeking to uncover the identities of human rights activists by actively attacking Google's and other people's corporate network.