I've done this with proper valid XHTML (the Google supplied code won't validate).

I've followed the advice regarding placement of the Google script tag, which they advise for reasons of performance (if the script tag was mid-page, it would have a small effect on page rendering speed).
However, it has not been hard-coded into FOOTER.tpl (which would have been a terrible assumption - not all screens/sites will have the button). Instead, I've added a bit of extra syntax to Tempcode to allow a template (BLOCK_MAIN_SCREEN_ACTIONS.tpl) to defer a bit of it's code down to the footer.

Actual placement of the button was almost trivial. It was about 1px too high in the template, so I made a wrapper div. That means ideally (to make things look perfect) this needs adding to global.css:


#screen_actions .google_plusone {
margin-top: 1px;
}