I doubt that the true unaffiliated percentage has changed that much over such a short time period. I think these numbers actually reflect people's willingness to reveal that they're unaffiliated, in other words, to publicly "come out" as atheists or agnostics.
Don't get me wrong -- I see this as a good thing overall, I just think the reason for the change is not what it seems. The first step is to discover religion has nothing to offer an intelligent person. The second step is to be willing to let your neighbors know what you've decided, to "come out". The second step is often much harder.
But the big change is still a long way off -- the day an atheist can run for president without being laughed off the stage.