If you are using traditional chinese (or non-english) client, the default setting of the launcher might not link to the us server(only the us server now available for updating).
1.Win7/ Vista:
C:\ProgramData\Battle.net\Agent
WinXP:
C:\Documents and Settings\All Users\Application Data\Battle.net\Agent
2.Find agent.db, open with wordpad (MUST BE WORDPAD)
3.Search: "http://zhTW.patch.battle.net:1119/patch" (this is for traditional chinese client)
change to: "http://enUS.patch.battle.net:1119/patch"