This URI <xmpp:jabber@conference.jabber.org?join> is an instance of the format documented here <http://www.xmpp.org/extensions/xep-0045.html#registrar-querytypes> and the registered handler for the xmpp scheme should join the chat room denoted by that JID.
In Adium 1.2 it opens what appears to be a vanilla Jabber IM chat with that JID and displays a transcript of prior conversation. It will do that even if you already are in the chat room. It doesn't show the room's roster, and it doesn't display the muc command menu. If you remove ?join from the URI it does something else.
No doubt this has entanglements in the scheme for creating bookmarks to chatrooms.