Microsoft provides access to Skype service in China through special TOM-Skype client. It is modified version of the Skype application which complies with local regulations. As it happens this version receives a list of keywords and triggers a spy mode when any of them encountered.
Post graduate student from New Mexico University discovered exact list of keywords and published an article about his research.
We would recommend not to use TOM-Skype client at all and use VPN while chatting with Skype in China.