Pseudononymous writing has been part of our culture for as long as the written word existed. Text by “anonymous” existed even in Ancient Greece. A detective in Britain lost a suit wherein he tried to stop a newspaper from revealing his true identity. The blogger wrote about the local police department and won a prestigious reporting prize for political writing. Since his outing, the blogger deleted his blog and was subject to a written reprimand from his superiors.
My understanding is that British and US laws are very different on this subject but I doubt that even a US blogger could get an injunction to prevent a newspaper from revealing a blogger’s identity. There have been cases which prevent ISPs from giving up identifying information, however; and there is a long history of US caselaw supporting anonymous speech.
(Thanks to Anion the Evil for bringing this to my attention).