This is a beta announcement of what I've been working on for the past couple of weeks (modulo firefighting!)
Open up any jabber/XMPP client (I use pidgin, but have tested it with a few others) and use your fastmail login/password (including the domain).
Most clients correctly do SRV lookups to find the server. If they don't, just set your server to 'chat.messagingengine.com'.
We have federation enabled, so you should be able to chat to any other XMPP user in the world - I've tested with Google Talk and Livejournal, because I have usernames there too. So this means you can chat with your own FM username with no extra effort.
Questions:
will it support aliases? I'm not sure yet. Have to look at issues like how to store the roster - is it shared or not? A bit tricky.
Group chat? I haven't tested that yet. This is a beta after all
Anyway - have a play, let me know what you think. I'd like to have a few more users on there to iron out any more bugs before we advertise it to everyone, so forum users, here's your chance to be early adopters again!