leaving a +1 for gtalk. so far it has worked flawlessly for me.
tried jabber with facebook chat just to test it since i loathe facebook, but it worked fine.
nice info on xmpp/jabber. have to look further into it.
maybe the n900 IM success for some is not the best but for me it has been nothing short of impressive. being online on several IM services and talking to several people in parallel, all integrated, gives a nice sense of freedom. :-) yes, the n900 is great still today.