WebMention is a modern re-implementation of PingBack, which uses only HTTP and x-www-urlencoded content rather than infinitely more complicated, not to mention bloated, XMLRPC requests. It was developed by the #indieweb community at IndieWebCamp, and is rapidly seeing adoption.
Since the best way to understand a protocol is to write an implementation of it, I bashed together a basic implementation of it for Elgg.
The plugin will automatically send webmention pings for content with URLs in the $object->description field (you can easily expand on this), it also exposes a webmention endpoint, and sets the appropriate discovery headers and meta tags. Plugin authors can hook into the plugin hooks that elgg-webmention generates and handle incoming mentions appropriately.
There is still a little more to do, the next step I think is to hook into a microformats parser, in order to get some richer semantic information as to the type of mention one is generating. My friend Ben has a very neat video of this kind of thing in action, and his idno project already implements it in the core code.
Have a play!