It seems that as of today, the Chinese authorities have blocked internet access to Twitter, Flickr, Bing, Live.com, Hotmail.com and several other sites. Wordpress, YouTube, Blogger are also blocked.

According to early reports on Twitter and on blogs it seems that the Chinese authorities want to quiet down the entire major social networking and social media part of the web ahead of the 20th anniversary of Tiananmen massacre on June 4th.

As far as solutions for evading the block go, you can find some advice here and here. Furthermore, Twitter may be working if you're using third party apps to access it, such as TweetDeck.