Some thoughts on security of IM apps.
MSN messenger is not very secure, it even tells you that every time you start a conversation. Yahoo Messenger is similarly insecure AFAICS. I would use another app, frankly. Trillian supports Secure IM over AIM & ICQ, which coupled with the ability to hide your IP, makes it a fairly secure solution. BTW, Trillian 2 has been released recently.
Integrity Messenger makes claims about being secure, although it's not immediately obvious from their site how they have implemented the security. I installed it, although I wasn't too keen on the interface, even though it worked well. On the up side, Integrity is be integrated with Invision Power Board, and there is a free version!
Integrity Messenger is available at:
http://www.integritymessenger.com/