I am now batting nodes, complete with imagefield, between separate instances of Drupal with merriment and glee. For yes, I do have a working beta of Content distribution.
I'm about to quickly write a service to get CCK's content_fields() array from the remote, distributing site, so that the retrieving site can show a UI of possible values for a Views nodereference argument. Try it. It all makes sense once you do.
But I remember a couple of months ago, when Services was a total mystery to me. I installed it, knowing that this was what I needed to achieve this task, but I was baffled by it. What was a Service? And how come I needed a server as well? Surely a service is a kind of server? Argh! And so on.
The penny dropped a few weeks ago, when I suddenly realized that a service is really just another kind of hook_menu() item.
Consider: in hook_menu(), you define which function Drupal should call when it gets an HTTP request at a certain path, say, 'node/1'. And in hook_service(), you define which function Drupal should call when it gets a service method request, in this case via XMLRPC (which is still black magic to me, but with Services module, I don't need to know, because it Just Works). For that matter, in Drush's hook_drush_command(), you define... etc etc. They are all the same!
This has me wondering whether for Drupal 8 we should abstract this out to a general hook_callback(), where we define what function Drupal should call when it receives any kind of external input.