Scott Schecter is on the hunt:

It's a bit of a hack in its current format, and what a surprise MSN doesn't play nice, but instructions can be found here on how to use Google Talk as a universal IM client*. It basically exposes Jabber transports for the other protocols for functionality. Cool, but I think I'll keep hunting

*The universal IM is an elusive little critter that is believed to live in Canada somewhere. It talks to all IM networks, but does not communicate effectively with any, you have been warned.