From Inside Facebook, “Facebook chat launching in the coming weeks. We’re opening up a new communication channel and enable real time conversation on the site.”
The Chat UI appears at the bottom; looks and works pretty similar to Gmail Chat. You can also pop conversations out into new windows, and possibly in the future there will be Jabber support as well as on-site messaging.
If Facebook could exploit the contents of your MSN/Yahoo/AIM address fields to build a seamless interface to these IM systems but using Facebook friend list, this would be an instant killer app. As it stands, I reckon Facebook IM would probably displace a good proportion of my IM usage.