Older, but very interesting: Rogers Cadenhead demonstrates, what you can find out about a person just from comments on websites. He only uses the comments on Metafilter to create a profile of a regular Metafilter user.
One should indeed think about what data and information one voluntarily gives away - bloggers and frequent commentators are often quite generous with their information. Efficient search engines can certainly help create a profile of a person that can be far more detailed than what Rogers did on Metafilter.