I tend to agree most with Pedro Melo in the sense that XMPP can simply be the transport mechanism and that the "higher" functions, if you will, of authorizing and subsetting followers can be done in an enclosing layer.
This has several benefits, including being able to gang or parallelize (yeah, that's not a word :) xmpp servers to distribute load from power-users and popular sites or to provide transparent back-up if your main XMPP server goes down. I think it's inadvisable to look at microblogging as only what XMPP provides, for instance using xmpp resources as the user-visible identities. JoeC
