To change the default (Traditional Chinese) fonts on Mac OS X, one may edit the following file directly:
/System/Library/Frameworks/ApplicationServices.framework/Frameworks/CoreText.framework/Resources/DefaultFontFallbacks.plist
Also note that to be able to modify this file, you need superuser right. So one can "sudo vi" to edit this file. I used STSong for monospace and san-serif, while STFangsong for serif. These fonts look better to me than those Hei type fonts.
A simpler way to do this is to use TCFail. Indeed, I noticed the path to the above file by reading the source code of TCFail.
Update 2010-11-16: To make this works, the order of the languages has to be changed, so that Chinese has to be put in front of other languages (e.g. Japanese). This can be done in the Language pane of the Language & Text panel under System Preferences.
