The free and open source Off-the-Record Messaging program offers a proxy for iChat that allows encrypted private messages, without requiring a .Mac account.
There's a plugin for Gaim, too, and support for other platforms and chat programs (Trillian, Adium, etc).
[robg adds: I haven't tested this one...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060215084902839