I have just logged into my control panel and enabled it (easy to do, they provide idiot-proof instructions) and that’s about it. From now on all invoices that go out will include the Google Checkout link alongside the Paypal (spit!) link.
I am impressed that I single handedly managed to get them to do this, with no help from anyone, they must think really highly of me! Just kidding, there has been a bit of a push on their forums for some time, and in the grand scheme of things it really didn’t take them that long to implement.
Great work guys, your work is really appreciated!