encrypted chat on the mac
The only real option for encrypted chat on Mac OS X right now seems to be Adium. The current version is Adium 1.1.4, which supports only one form of encryption, "Off The Record" (OTR), unless there is some plugin I'm overlooking. The upcoming version (2) of Adium will use libpurple, which is behind Pidgin (formerly known as Gaim); possibly this shared code will help make more encryption options available.
I've also seen that iChat supports encryption, but only between .Mac subscribers.