This is disturbing and has accelerated since the mid 80's

The percentage of people who do not pay federal income taxes, and who are not claimed as dependents by someone who does pay them, jumped from 14.8 percent in 1984 to 49.5 percent in 2009.”
That means 151.7 million Americans paid nothing in 2009. By comparison, 34.8 million tax filers paid no taxes in 1984.
http://blog.heritage.org/2012/02/19/...-income-taxes/