Thanks, witchspace. I agree too that XMLRPC is a bloated monster. However, I've been working with the big 3 FOSS CMS systems this past couple of weeks and, looking at what comes bundled with them, find:
WordPress: xmlrpc, no jsonrpc
Drupal: xmlrpc, no jsonrpc
Joomla!: xmlrpc, no jsonrpc
This isn't a huge deal since there's a very functional PHP library available (http://phpxmlrpc.sourceforge.net/
) which supports JSON-RPC as both client and server.
The request for that feature boils down to "follow what's popular rather than what's better, in order to drive adoption". I really should have made it #2 since the callback capabilities are far more important.