It's also interesting that while the numbers of atheists and agnostics HAVE increased, the vast majority of growth for the "Unaffiliated" category comes from those who are not affiliated with an organized faith but also are not atheists (presumably with a faith in some higher power but no religion).
As RINO Tom pointed out, though, let's not confuse "Unaffiliated" with being an atheist, or even an agnostic.
"Nothing in particular" could be a great many things, including a form of atheism and agnosticism. I describe myself as 'apatheist', for example.
I'm inclined to believe that if one were an atheist or an agnostic, he or she would have marked that, seeing as it was one of the available options ... I know quite a few people who are more or less sure that they believe in a "Creator" of sorts, but totally reject religion. I'd bet good money that's the largest group within "Unaffiliated."