41% of Americans don't pay federal income tax

At a conference today Chric DeMuth of AEI mentioned that a large fraction of Americans pay zero federal income taxes. I googled, and found that a reasonable estimate is 41% of households (from http://www.taxfoundation.org/research/show/1410.html ). That's 43 million tax return (for 2006) out of 136 million total, with an estimated 15 million that don't file tax returns at all.