Putnam and Campbell, in American Grace, offer a pretty high number for Americans who say they have no religion: 17%.
As others have found before, Putnam and Campbell find that the Nones tend to be young, liberal, from unchurched, mainline Protestant, or Catholic homes. And they are very changeable - most come from a churched background, and many will end up churched later in life.
One interesting new finding is that many of them change their self-definition without changing their practice. Even in the year between the two iterations of their Faith Matters survey, 30% of the people in the category had changed. Many of them said "no religion" one time, and named the tradition they came from or were heading to the other time.
The authors conclude that many of the Nones are not anti-religious, and only a tiny fraction are atheists. Rather, Putnam and Campbell see that around each major religious family there is a "penumbra" of an additional 10% who sometimes see themselves in the fold, and sometimes see themselves outside of it. The people who say they have no religion are not, for the most part, anti-religious, but are disappointed with the religious institutions they know - and many would like to find a way to come back.