Now that the Census is upon us, it's interesting to look at its mention in the U.S. Constitution.
From Article 1, Section 2:
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers...The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct."
The original purpose of the Census was as a headcount. Its modern function includes an attempt to statistically analyze the U.S. population. The 'long forms' are especially intrusive. And, extremely inaccurate given that a substantial chunk of the population isn't interested in giving out that kind of information. One can look at the inaccuracy of exit polling, over the last few election cycles, to see how self-selection can skew results. Add to this the politicized context of the Census at the endpoint of interpretation. Though it is supposed to include everyone, on my experience with long forms in 2000, on door to door follow-ups there are plenty of gaps in information and people unwilling to share. This is why Census workers are passing out "Your Answers are Confidential" notifications right now.
But, problems with privacy wouldn't be with the Census worker at the door. It would be at the final destination of the information.