The OLPC will have built-in extended range low-power wifi networking capability. However, most of the laptops will operate outside the normal networking infrastructure that we take for granted. There won't be any coffee shops with wifi Internet access service. There won't be any kind of Internet access. There won't be landline telephones and there won't even be cellular in some regions. Therefore, the built in chat application will have to be innovative, in the same way that the hardware design is innovative. In fact, maybe we shouldn't even be calling it a chat application.
The pages listed below all explore some aspects of messaging and offer ideas that could be incorporated into the OLPC.