I'm guessing this report comes out annually every January for the previous year but it might be something you want to reference for future discussions in this forum about union membership seeing as how they have a lot of clout under Obama. It includes demographics about union members as well as data by state. There are links in the Bureau of Labor Statistics press release to various tables.
"In 2009, the union membership rate--the percent of wage and salary workers who were members of a union--was 12.3 percent, essentially unchanged from 12.4 percent a year earlier, the U.S. Bureau of Labor Statistics reported today. The number of wage and salary workers belonging to unions declined by 771,000 to 15.3 million, largely reflecting the overall drop in employment due to the recession. In 1983, the first year for which comparable union data are available, the union membership rate was 20.1 percent, and there were 17.7 million union workers."
Union Members Summary