I sometimes wonder why people with poor or limited network connections don't still use UUCP email. It can be configured to use precious bandwidth during off-peak times, handles slow networks and unreliable communication. This would seem to be the perfect piece of infrastructure to be rolled out with the OLPC project. If your network does not have a wired connection to the world is even works over HF radio.
All you have to remember is that email is not an instant messaging medium.