LiveJournal has just announced its own Jabber-based instant messaging service. What this means is that, if you have a Jabber client (i.e., Adium/iChat on Mac, Gaim on Linux or Trillian on Linux), you can connect to LJ's servers using your LJ username and password; your friends list will appear in your contacts and be contactable. And since it's Jabber, you can send messages to/from other open Jabber systems, such as Google Talk.
It's currently in beta pre-alpha. Apparently once it['s working, there are plans to implement an AJAX client, much like Gmail's chat mode.