Thailand will conduct the next national census - including both housing and population - in 2010. The ten year rhythm was adopted in 1970, following the United Nations recommendation to hold the national census in each year ending with zero. This forthcoming census will be the 11th census, earlier ones where done in 1909, 1919, 1927, 1937, 1947, and then since 1960 every 10 years. Also since 1960, the National Statistics Office (NSO, สำนักงานสถิติแห่งชาติ) is responsible to take out the census. Some more background on the forthcoming census compared to the previous ones can be found in a paper by Jirawan Boonperm presented at an ASEAN meeting in 2006.
The results of the 2000 census are available on the NSO website, partially as PDF documents, partially as Excel sheets. Note that even the NSO admits that these data have their problems, especially the population number in Bangkok is in real higher than counted - but in fact the population number by the Department of Provincial Administration (DOPA) is even half a million lower than the 6.3 million counted in the 2000 census.