The Bloggers' FAQ on Privacy addresses the legal issues surrounding the privacy rights of people you blog about.
What are private facts?
Private facts are personal details about someone that have not been disclosed to the public. A person's sexual orientation, a sex-change operation, and a private romantic encounter could all be private facts. Once publicly disclosed by that person, however, they move into the public domain.
Can I be sued for publishing somebody else's private facts?
Some jurisdictions allow lawsuits for the publication of private facts. In California, for example, the elements are (1) public disclosure; (2) of a private fact; (3) that is offensive to a reasonable person; and (4) which is not a legitimate matter of public concern. Publication on a blog would generally be considered public disclosure. However, if a private fact is deemed "newsworthy," it may be legal to print it even if it might be considered "offensive to a reasonable person."
What is offensive to a reasonable person?
To state a claim, the plaintiff must show that the matter made public was one that would be offensive and objectionable to a reasonable person of ordinary sensibilities. For example, disclosing that the plaintiff returned $240,000 he found on the street was held not to be offensive, but the publication of an "upskirt" photo would likely be found to be offensive to a reasonable person.
How do I know if a private fact is "newsworthy"?
A private fact is newsworthy if some reasonable members of the community could entertain a legitimate interest in it. Courts generally recognize that the public has a legitimate interest in almost all recent events, even if it involves private information about participants, as well as a legitimate interest in the private lives of prominent or notorious figures (such as actors, actresses, professional athletes, public officers, noted inventors, or war heroes). Newsworthiness is not limited to reports of current events, but extends to articles for the purposes of education, amusement, or enlightenment. However, a court may look at whether the private fact is pertinent to an otherwise newsworthy story.
What is "intrusion into seclusion"?
Intrusion into seclusion occurs when you intrude upon the solitude or seclusion of another person or his private affairs or concerns, if the intrusion would be highly offensive to a reasonable person. It generally comes up in the context of paparazzi photographing celebrities, but covers any reasonable expectation of privacy that is intruded upon. If the person intruded upon gave you consent to do it —i.e., gave you permission to take his picture or write about him —then you have a defense against this claim.
Interception of an electronic communication (i.e., an email or IM chat) can raise additional legal issues, such as federal wiretap laws.