To quote from their announcement, "The Integrated Public Use Microdata Sample (IPUMS), created at the Social History Research Laboratory of the University of Minnesota, is now available. The IPUMS consists of 23 samples of the U.S. Census. Together they comprise our richest source of quantitative information on long-term changes in the American population. The IPUMS combines these high-precision samples of the U.S. population into a single database spanning eleven census years from 1850 to 1990. The database includes over 15 million person records (soon to be over 50 million). Most of the census samples have existed for several years, but because the original samples were coded and formatted differently, they have been very difficult to use in combination. The IPUMS assigns uniform codes across census years and integrates the documentation into a coherent form."
The 800 page user guide can be downloaded from their site, or you can order it for $30. The actual data is available from their site as well. It varies in size, as you'd expect. The 1850 Census sample (compressed) is 6.9 meg in size, while the 1% 1990 sample (again compressed) is 163 meg in size.
A revised version is expected in the Fall of 1996.
http://www.hist.umn.edu