The official U.S. Census is described in Article I, Section 2 of the Constitution of the United States. It calls for an actual enumeration of the people every ten years, to be used for apportionment of seats in the House of Representatives among the states. The first official Census was conducted in 1790 under Thomas Jefferson, who was the Secretary of State. That census, taken by U.S. marshals on horseback, counted 3.9 million inhabitants. Since that time, the decennial Census has been conducted every ten years, generally on April 1 in years ending in a zero.
All the available census records in the USA are open to the public until the 1930 Census. The 1940 Census will be released in 2012, the 1950 Census will be released in 2022, the 1960 Census will be released in 2032, etc.
Why? Because the Census contains personal information, and it cannot by law be released until the information is 72 years old (roughly the life expectancy in the USA, or at least it used to be). The personal information contains facts such as household salary, names and ages and occupations of everyone in the household,
I think you may have gotten confused because the general facts for all censuses are available at http://www.census.gov/. In other words, if you're looking for a population of a city, or the median income for a zip code, etc., that information IS available to the public, currently up through the 2000 Census. This general data is very useful if you are performing market research for, as an example, a store you want to open in a specific city. It has many, many uses.
I hope that helps!