The current blocking may be related to the recent anti-China protests of Beijing Olympic Games, which began on 8 August in Beijing. Some days before, the author of a very popular free source code editor in SourceForge named Notepad++ start to boycott Beijing 2008, he said that the action is not against Chinese people, but against Chinese government's repression in 2008 Tibetan unrest. The boycotting is surely a politic issue, but refers to human rights problems in China.
Is that the reason of this blocking? Chinese authorities regularly blocks access to websites that it finds objectionable, including those dealing with politically sensitive subjects. That's what we have been. Blocked.