The National Institute for Early Education Research supports early childhood education initiatives by providing objective, nonpartisan information based on research. It is a joint effort of the Pew Charitable Trusts and Rutgers University.
There have been 5 reports on the State of Preschool in the United States.
All the reports , including the current one (2007) are available at http://nieer.org/yearbook/
The site also contains interactive data with data sets, state statistics, and graphs.