Unfortunately Twitter in China is blocked by the Great Firewall, so if you are in China and want to Micro Blog, you should choose from one of the local alternatives.
Previously there were several other local alternatives such as Fanfou, Digu and Jiwai, but all of these are either under “server maintenance” or simply offline.
UPDATE: SINA just launched an online twitter service that should never be blocked by the great firewall. t.sina.com.