In case you missed it there has been a change made by the Theme Team to the display of Author bylines on WordPress.com free hosted blogs. Previously, there were theme specific variations. Many but not all theme displayed an Author author byline in the post metadata. Some themes displayed Author bylines and some also displayed Author profiles below their posts. Regardless of the theme, bylines will only display now if there are at least two authors who both have at least one published post in the blog. This post announced several theme enhancements from November 2011 and includes why when there is only one Author publishing in a WordPress.com blog you are no longer seeing Author bylines.
Our most popular themes now have better comment labeling when comments are turned off and better author highlighting for multiple author blogs — making them perfect for blogs that are more like small business sites or magazines. (Italics added.)
Work-around for single Author bloggers who do wish to have a byline display on their posts
While logged out of WordPress.com create another username account with a different email address. Then add the new username as an Author. Then log-in as the new author and publish one post and backdate it to the date you started the blog. Log-out and clear your browser cache and cookies.
Log in under the original username and you will have an Author byline.
In some themes there will also be an Author Profile displayed below each post. The data for the Author byline author profile is from this page > Users > My Profile > “About you” section . If you do not wish to have an Author Profile appear then remove the information you previously provided on this page > Users > My Profile in the “About you” section and click “Update Profile”.