The use of subpoenas to force journalists to disclose their confidential news sources and unpublished information significantly intrudes on the newsgathering process. Shield laws exist in forty states (W. Va. passed one in April 2011); if a reporter isn't covered by a shield law, there may still be a constitutional privilege that helps protect sources and information.
Under either scenario, the right is often 'qualified" -- balanced against other interests -- and there are still exceptions, such as when the reporter is an eyewitness to a crime. Digital journalists are often covered by the privilege. All journalists may also have legal obligations to their confidential sources if they breach the agreement. Web sites may be able to protect the identity of anonymous posters, under developing laws or even shield laws.
If you are subpoenaed, there are certain steps you should take immediately. Separation orders can keep you from attending a trial if you are subpoenaed. Reporters should also be aware of the sanctions for non-compliance. Federal law and some state laws protect journalists from having their work product and documents seized without a warrant.