It's an XMPP extension.

Even if I agree somewhat with your feelings about xml, I think I prefer something like xml over (a sample) of: RFC 1939 POP3 protocol Updated by RFC 2449 RFC 2449 POP3 Extension Mechanism RFC 822 STANDARD FOR THE FORMAT OF ARPA INTERNET TEXT MESSAGES Obsoleted by RFC 2822 RFC 2822 Internet Message Format RFC 2045 MIME Part One: Format of Internet Message Bodies Updated by RFC 2231 RFC 2046 MIME Part Two: Media Types RFC 2047 MIME Part Three: Message Header Extensions for Non-ASCII Text Updated by RFC 2231 RFC 2183 The Content-Disposition Header Field Updated by RFC 2231 RFC 2231 MIME Parameter Value and Encoded Word Extensions: Character Sets,... RFC 2387 The MIME Multipart/Related Content-type RFC 3462 The MIME Multipart/Report Content-type RFC 2111 Content-ID and Message-ID Uniform Resource Locators RFC 2632 S/MIME Version 3 Certificate Handling RFC 2633 S/MIME Version 3 Message Specification RFC 2821 Simple Mail Transfer Protocol

Yeah, me too, but writing a bi-directional wrapper for JSON or the even better future thing might be non-trivial -- unless the protocol gets ugly and uglier.

Yeah, it looks a lot like XMPP to me, too:

  "The Google Wave Federation Protocol is an open extension to XMPP core [RFC3920] protocol..."

(From the first paragraph of the draft spec.)

Whatever your feelings about XMPP, I challenge you to point to a standardized, interoperable protocol that you think would be a better fit. It may not be ideal, but it's extensible, already supported by a large number of clients and servers, and doesn't require starting from scratch when building interoperable software.

I think there is room for something like AMQP. Most people use libraries to process XMPP anyway, so a binary format wouldn't be a huge change (except, perhaps, in performance).

XML is not suitable for Mobile Platform.