Information collected by the U.S. Census Bureau is kept strictly confidential for 72 years. During that 72-year period, statistical data for states, cities, and census tracts are published, but not the personal details about individual people. However, after the expiration of the 72-year confidentiality blackout, every last detail is open for everyone to see.
The wraps are now off the 1940 census.
It is not the Census Bureau itself, but a separate Federal agency, the National Archives (more formally: the U.S. National Archives and Records Administration), that provides access to the data, via the following Web site: http://1940census.archives.gov/
(The actual Web hosting, incidentally, is by a private company whose Web site is archives.com .)
The data are organized geographically, not by people's names. Private organizations will index the data by name and make the indexed data available at a later date (probably late this year), usually for a fee, for genealogical research. (According to the National Archives, you can get free access to certain services, such as ancestry.com, at regional National Archives offices or at certain libraries.)
If you know someone who is over 72 years old, and you know where they lived as of April 1, 1940, then you may be able to find their census records from 1940. (1930 census records are available online, too.) Or, if you are just curious to see what you might find out about a particular house, street, neighborhood, small town, or prison, then you can just go knock yourself out and snoop to your heart's content.
The 1940 census collected more information than did the 2010 census, for example: state or country of birth, level of education, citizenship, occupation, annual income, number of weeks worked in the previous year, and number of hours worked in the last week of March. Truly a treasure trove of trivia.
The records that are now available are the original handwritten forms. Apparently, penmanship had not yet evolved to the modern, more legible, "block printing" style, so you have to be able to read cursive writing.
The process for looking up Census records is somewhat cumbersome. First, you must determine the "enumeration district" (jokes about "E.D." left as an exercise) corresponding to the address that you want to look up. Then, you can find the set of census records for that ED. There is no particular order to the records other than the order in which the information was collected. (It was not until 1960 that census forms were mailed out, so all the information in the 1940 census was collected by the census taker.) Consecutive house numbers are usually, but not necessarily, adjacent. You may have to scan through perhaps 20-40 pages to find the address that you are looking for.
Just figuring out the ED number can be a bit of a puzzle. At the very least, you'll need the county name for the address you are interested in. But you also will want to look up the address on Google Maps so that you can find the nearest cross streets. You may need to locate the block on a Census map that illustrates the CDs for a city, or you may need to read through a document that describes each ED by listing the street names for its boundary. It may be a nuisance, but: no whining, please, because, keep in mind, no one wants to hear about your ED problems.
Helpful instructions are at: http://www.archives.gov/research/census/1940/start-research.html
At the end of the helpful instructions, they refer you to the utilities provided by some guy named Steve Morse: http://stevemorse.org/census/unified.html
I found both the official National Archives pages and Steve's unofficial utilities to be useful in looking up EDs.
The Web page that you will usually start working from is: http://1940census.archives.gov/getting-started/
To see the census records, enter the state and ED number (in the section labelled "Do you know the 1940 enumeration district number?"), click on "View It", then (when the results appear) click on "Census schedules", and then click on the "View ED <ED number>" link.
The Web software that displays the census pages is currently only semi-functional. They are working on it. But I will describe what currently works and does not work. Currently, there is no way to zoom. The display is zoomed all the way in, so you can only only a very small portion of the page. (Yesterday, you could zoom, but there was never any actual image to zoom in and out of; you can't have everything.) The print feature seems to be broken. The download feature does let you get a full-size image of the entire page, but the feature to download all the pages for the ED seems not to work.
You have to scroll through each page to find the address that you are looking for. First, look for the desired street name. The street name is written vertically in column 1, so you need to turn your head to the left to do this. If you find the desired street, turn your head back to its original upright position, and then scroll through column 2 to find the house number. Apartment numbers are not recorded.
When you find the page you want, use the download feature to display the full image for the page (standard resolution), and save the image to your computer. The full-size image is more useful for printing, but, if you want to send it in email or put it on a Web page, you will probably want to reduce the size by about 25%.
Some interesting information that you might miss if you don't go looking for it: column 5 has the monthly rent or value of an owned house, column 14 has the highest grade of school completed, column 26 has the number of hours worked in the last week of March, 1940, column 31 has the number of weeks worked in the year 1939, and column 32 has annual income.
A sample of about 5% of the population, selected pseudo-randomly by lines 14 and 29 on the 40-line form, was subject to supplementary questions, recorded at the bottom of each page.
If you see a circled (X) after a person's name, it indicates that this is the person who provided the information for the household.
I was successfully able to find census records for my grandparents, for the grandparents of a friend of mine, and for some folks named Franklin and Eleanor who were living at 1600 Pennsylvania Ave, Washington, DC.