I just picked up a (cheap virtual) server and the xochat.org domain with the intention of running a community Jabber server for XO users. That is, non-local chat and, I think presence, on the XO is done via the open Jabber or XMPP protocol. Schools will have their own Jabber server on their school server, OLPC has their own, but they're not really up for the task of maintaining a server with hundreds of thousands of users. I'm not really either, but I should be able to handle a smaller load. Also, this is strictly an experiment. I make no guarantees it will work.
For that matter, I should make no guarantees it will even get off the ground, notwithstanding that I've now invested $60 in the effort and written this blog post, which kind of puts me on the hook. But hopefully I'll get the configuration right and once I get my XO and figure out how to do it, I'll tell you how to connect to my server. I don't think it will strictly matter which server you use -- you should be able to chat with people using different servers, but I'm not 100% sure. Figuring out how it actually works is part of the motivation here.
Also, the XO's use the Jabber PubSub extension which, as I've written in the past, could reasonably used as a replacement for SIF's oddball architecture in doing various sorts of data integration within schools, so I have a professional interest in working through the possibilities there in more detail, now that it looks like there will suddenly be a lot of schools running Jabber PubSub servers.
Also, if you've got a Mac OS X server at your school, you can turn on a Jabber server with a few mouse clicks, although it won't have all the same extensions the XO's are using, so ymmv.