Thanks to delphinios for pointing this out, you can now instant messenge using LJ Talk, which uses the Jabber messaging protocol. Just fire up any client that supports Jabber (such as Trillian) and read the following to set yourself up.
The coolest things about LJ Talk? Adding this connection will automatically add your LJ friends! Also, you can even post to your journal using this service (just message "(LJ Bot) Frank" on your list with "help" for instructions). Oh, and of course, it's totally free. Whee!