I put the +1 button on my site today.
Google have recommended putting this non-asynchronous code for the +1 button at the start of your page code (at the top) near or in the <head> my recommendation is to do the reverse, put it in the footer or near the end of the code. The button will appear as one of the last things on your page, but importantly it will not interfere with your content loading - which the user is most interested in seeing first.
You may disagree but I found this helps my pages both 'load' and 'appear' to load quicker ... in essence:-
and this code where you want the button to appear: - <g:plusone size="medium"></g:plusone> note:change the "medium" to "small" or "tall" or delete it, for whatever button you want etc.