The main page of the Chinese-language version of Wikipedia (zh.wikipedia.org) could be displayed and searches for apolitical terms turned up results, but searches for subjects taboo to China's Communist leadership, such as "June 4", remained blocked.More info at Reuters.
June 4, 1989, was the date that China's military crushed a student-led movement for political change centred on Beijing's Tiananmen Square, killing hundreds and possibly thousands. The incident remains among the most sensitive subjects for the country's state-controlled media.
Update: I put this news story in the queue last week and it looks like it's already outdated. The ban was only lifted for a very short term, on Friday the Wikipedia site wasn't accessible anymore in China.