A few of the wp.com themes have a default display of biographical information entered on the Your Profile page which will appear in the default widget display at the top of your sidebar. However, the minute you choose to add or change widgets the default display will disappear.
Most themes do not have this display of the biographical information from the Your Profile page at all. When this is the case, or when you choose not to rely on the default widget this display of biographical information can be achieved in two ways:
(1) a text widget that contains a brief bio can be placed it in your sidebar, or
(2) you can create a static “About” page.
Also note that if you look up at the top right hand corner of the wordpress.com forum page you will find a Profile link.
Lastly, you can link your wordpress.com username to your wordpress.com blog
-> Dashboard -> Users -> Your Profile
scroll to “website” in the right hand column and insert the url for your wordpress.com blog and then click “Update Profile”.
To view and/or edit your own profile visit: http://wordpress.com/profile/
To view any wordpress.com user profile replace “username” with the appropriate one and visit: http://wordpress.com/profile/username