Google has now allowed to add Google Plus +1 button in WordPress sites easily.
Webmaster have been in hurry to add +1 button in their sites in hope that it might influence the Google search engine ranking and influence SEO.
Otto has posted a code to make a pulgin for WordPress. But, it is crude form yet.
Here is the easiest way of adding the button:
Visit this Google page. You will see the page as shown in the screenshot below. Choose the size of the button (preview is shown in box on the right). Scroll down and copy the code and paste on the header portion and the place you would like the button to appear.
The following video made by Google explains how Google +1 works: