This looks like it should enable server to server communication via HTTP
What you'd really want to see is the resful api working will with the MVC Controller, so I can send a request to almost any (for example user/wall etc.) page, and it will be able to pass it along to the right handler.
Global Idenitifiers (GUID)
Well they're actually local identifiers but unique to an install, so combining with the install root, they can become global. But every object has a global identifier.
I've reviewed it in more detail and I'm pretty impressed. I think at a minimum this is a project that much can be copied from. We can take a lot of the GNUv2 ideas and reuse, I think they've got a hell of a lot of stuff right. It's not quite as modern a framework as the likes of symfony, and the FOAFs need work, but there's a lot of work in there and plugins that would get us quite far quickly.
If we can get a federated social net obeying linked data principles under AGPL quickly nailed, that can give us more scope to adding fun stuff like realtime protocols, desktop integration, encryption etc.
Is there a case for branching elgg? Or perhaps beta testing a sample elgg community for a month or so, and see if there are any roadblocks.