Skype use in China was especially risky for a while -- when a local company (TOM Online) was processing data with no real safeguards, it virtually guaranteed the presence of government monitoring. Now that Microsoft has teamed with Guangming Founder on a new China-specific Skype build, there's a chance that your chats are more secure. GreatFire.org has noticed that the updated app encrypts all data and sends it directly to Microsoft, making it harder to censor or spy on any communications. Don't be too quick to discuss taboo topics, though. It's not clear that the Chinese software is free of back doors; Microsoft says it's following local regulations, which aren't exactly open-minded. Even so, anyone already using Skype in mainland China will likely want to upgrade if they value some semblance of privacy.
Update: Guangming Founder was created by Guangming Daily, a government-backed newspaper; while Skype's new approach is still more secure, the association casts extra doubt on the privacy that users can expect.