I set up Proteus to access the Google Talk IM system. Google Talk appears a service that is intended to compete with AIM and Yahoo IM. It uses the Jabber open standard for communication. The instructions that were posted at TUAW and Make: Blog worked perfectly for Proteus.
I can't tell if Google is allowing communication from Google Talk users with other users of other Jabber servers. Ideally, they would allow this since this is one of the benefits of using an open standard.
I do wonder about the need for another IM service. Currently all of my contacts use AIM or Yahoo IM. I can see the benefit of this service if people start to use it.
If you would like to say hello, I can be reached at firstname.lastname@example.org on Google Talk