There is a working prototype of the chat infrastructure in CVS. There is currently no nifty front-end like IRC or Jabber for it. Feel free to write one. In the meantime, you can grab it from CVS, compile it, and run eof.chat.TestClient to try it out. It will log you onto a channel, print messages it receives, and say anything you type to the channel. Type "/quit" to quit.
Chatting is pretty slow even for two local users. There's also some startup time needed to join a channel as your client has to download all of the old messages in order to get the current message index.
In case you're wondering, yes, we know that chatting over Freenet is a fairly insane idea due to the lag. We're going to do it anyway because we are in fact insane.