Quick Tip: Enable iChat Encryption for Private Chats
Most of the time what I say on iChat is unimportant and would bore any eavesdropper, but on occasion I need to communicate some sensitive info, and I feel uncomfortable doing that over an unsecured connection. Luckily, with MobileMe there's a way to make it secure.
Source: http://gigaom.com/apple/quick-tip-enable-ichat-encryption-for-private-chats/