I've been looking at this from the perspective of adding support for this in some GNOME Do plugins. gnome-online-accounts is basically a single-sign-on API. It provides a way for applications to get an authentication cookie for the configured online accounts. It does not provide anything else; it's up to the application to know how to use that cookie to talk to the online service.

As such, it's entirely dependent on the applications to (a) query gnome-online-accounts for configured online services, and (b) actually know how to interact with those online services.

You can think of it as basically gnome-keyring for your online services. It has been integrated into Ubuntu 11.10, in so much as the GNOME apps which support g-o-a will use it; there just aren't many apps that use it yet ☺.