Your plugin inserts the google code just fine, but.. I have a problem.
I created an experiment with two pages. The pages have the same title ('Title') but different content ('Text A' and 'Text B'). The code works and Google serves me version B. The problem I have is that my 'Recent posts' and category pages now show two similar posts, and visitors can choose which version they want. Besides that is looks stupid, it also ruins the experiment.
Is this something your plugin was intended to deal with, or should I look for a workaround?