Adium connects to facebook chat the same way meebo and pidgin does. They uses something called libpurple.
At this time you cannot get your facebook chat in iChat, but i’m sure it will be soon as ichat has jabber support.
Well, libpurple is the base of both adium and pidgin, but ichat is coded in cocoa and cocoa plays nicely with stuff in general, so who knows. Maybe someone could use some of the code in libpurple to create a plugin for ichat.