The link below (which I also posted further down in the thread) is a blog post by a member of the Tax Policy Center, which did the research. It says:
It is no accident, btw, that the number of people not paying income tax was so high in 2009. You may have noticed that we’ve had a recession lately. And here is a powerful insight: When people’s incomes decline so too does their income tax (at least most of the time). At the same time, many working families have benefited from temporary tax cuts aimed at boosting the economy, and as a result some did not pay income taxes last year. As the economy improves and those tax cuts expire, it should also be no surprise that the share of people who don't pay income taxes will likely shrink from half last year to less than 40 percent by 2012.