imagine if i could login at antville.org as tobi/p3k.org and the antville installation there would retrieve the user session via xml-rpc from this antville installation here... theoretically, this way it would be possible to login at any antville site.
(with the right api definition this even could work between any two blogging systems.)
however, right now i only can guess that there is at least one big problem with such kind of remote login.
and it opens a bunch of questions, e.g. which site is a comment stored at? or: can i become a remote member of an antville.org site?