You can find the original announcement
here. Which includes a list of clients known to work. I've switched to
Pandion as a client, which seems to be a tad shinier and a little easier to use (not that there's much that can go wrong). I'll undoubtably try several out before settling on one.
The salient points are:
The server is livejournal.com
Jabber is designed to work with multiple servers - which is why my username is andrewducker@livejournal.com - that means that Google Chat users (which is also Jabber based) can contact me by adding that address to their contact ists (and vice versa). Google and Livejournal will then pass Jabber messages back and forth. This is true of any Jabber server, so people can feel free to add me from elsewhere.
Theoretically Jabber is the future of IM - it's an open, extensible protocol for Instant Messaging, where anyone can run a server. It's basically the IM equivalent of email or the web, unlike MSN, Yahoo or AIM, which are all proprietary networks owned end-to-end by a single corporation. Give it a go.