XMPP defines S <--> S as well, but Google hasn't implemented that on their server. What Miguel was talking about is clustering {S,S,S} <---> ... which (last time I worked on it) was pretty complicated thanks to the s2s dialback protocol (server A initiates a connection to server B using s2s, and server B does a DNS lookup of the domain server A and dials back the authoritative DNS response to authenticate server A). The s2s specification is what sets XMPP apart from MSN/Yahoo/AIM as it means any network can provide their own domain specific server and users on that networks can talk with any users on any other networks server without having multiple accounts (just like email!). We just have to wait for Google to turn it on.