iChat: Add new account in Preferences, select type Jabber, enter username "<username>@chat.facebook.com" (where username is selected in Facebook settings, and you need to logout from Facebook after that change). For password use Facebook password.
New account, select XMPP as protocol. Username is your username (which is your slug if you have a custom url. So I'm facebook.com/daniel.jackoway, my username is daniel.jackoway). Domain is chat.facebook.com. Password is your password. Only gotcha is that you then have to go to the "Advanced" tab and uncheck "Require SSL/TLS".
account add jabber FACEBOOK_USERNAME@chat.facebook.com FACEBOOK_PASS
account list
account add 2 (or whatever number the above list shows is Facebook)
# you'll need to do a /whois on your friends and rename them:
rename u393222111 John
rename u134322433 BobThe renaming is fairly annoying, can anyone think of a way to auto-name users?
Here's one: http://a.ngus.net/bitlbee_rename.pl
At first I didn't log out after setting my username and that made it hang. That step is important.