"..in short, don't worry about data representation and creating a standard way to represent all data (thus incurring ineffeciencies due to the necessary generality of this task), instead focus on providing easy ways for people to make your protocol actionable. that's what really matters. with technologies such as Java Reflections, now this protocol->method/action binding doesn't even have to be compile time, so there really is no argument against such an approach.
XML can be many standards in a way, No?
If there are going to be all these devices then I perceive XML as the only way to go because you can have diversity within the standard of XML. A very Rich protocol indeed! So by the time p2p goes mainstream within gadgets and processing is up to par, jXTA should be there to take control. ;-)