One of the requirements of owning a domain is that you must give your 'name, address, email, and phone number' while registering a domain. This rule was created to make it easy to contact a domain owner in case of problems, but it has since been exploited by spammers and marketers to bother people. Domain registrars offer "privacy or WHOIS guard, Private Domain Registration" that costs $8 a year.
Now, WordPress.com allows "adding "private registration" when you buy or have existing domain registered with them. If your domain expires in less than six months in current year you'll get rest of the year free but you'll have to renew domain and cover the cost of privacy upgrade for the remainder of domain's lifetime.
With a private ("unlisted") registration through Domains By Proxy (our affiliate company), registering a domain name doesn't mean sacrificing your privacy. Their name is shown on the registration instead of yours, but you still retain the full benefits of full registration," stated Donncha.