And it's based on Jabber, an open source instant messenging service, which is good because iChat AV for Mac is also based on Jabber. Here's how to get set up if you have a gmail account already and a Jabber client:
Server: talk.google.com
Username: youremail@gmail.com
Password: yourpassword
Let me know if you want/need a gmail account.
Nothing too exciting here - they've got a client for Windows folks to use - but I know how tied most people are into using MSN. It's similair to how everyone used to (still does?) use Internet Explorer just because it came installed on Windows.
