While we cannot yet publicize our posts to Google+ Pages (see this update), we can, however, add a link to either a Profile or a Page in a WordPress.com site’s sidebar by going to this Google page while logged in on Google Plus:
Because of code restrictions on WordPress.com, you can only use the Icon option offered (not the Badge as the code will be stripped out). Once you have created the Icon on that page, copy and paste the code you receive into a Text Widget you’ve added to your sidebar and save. Adding this to your sidebar will connect your site to your Page or Profile as the Publisher of your site.
More on Google Plus on WordPress.com: Google Plus