Probability distributions can be a funny thing. I read today that only three pieces of information: A zip code, date of birth, and gender, are enough to uniquely identify 87 percent of the U.S. population [see Cryptography: How to Keep Your Secrets Safe, page 5].
Consider that the New York Times requires almost this much information in its registration:
I'm not saying that the NYT is necessarily trying to identify its subscribers, I'm just using this as an example of what's statistically possible with a few innocuous-looking questions.
2008-12-04T12:02:54Z