To illustrate how the new Services API works I wrote a new "Example" feature for Aegir.
http://is.gd/d83Jl (ui) - http://is.gd/d83JZ (backend)
The front end contains a dummy hosting feature (enable under experimental), that when enabled exposes a new "example" service type.
When editing a server node, you will be able to enable the 'basic' implementation of this service type on that server.
The front end includes all the methods necessary to load, save and display the service information, including showing how to pass this
information to the backend to enable your new service.
The backend contains the new service type, with it's one basic implementation. All it really does is turn on when you enable it in the
front end, and saves the information you entered on the UI to the "~/.drush/$servername.drush.alias.inc" file. It also generates a
configuration file in "/var/aegir/config/$server/example.d/example.conf".
It implements most of what you need to get up and running with your own services (ie: DNS, SMTP, LDAP provisioning support etc.).
The basic implementation can also be used as a template for extending the existing db and http service types (for postgresql or nginx support for instance).