From the latest Pew poll on the subject:
One-third of Americans reject human evolution
On the flipside of course, 60% of Americas do believe humans evolved over time. Note that this figure does not distinguish between those that believe evolution is a purely natural phenomenon versus those that think that god had a hand in it.
I am not entirely positive and perhaps the other forum members can provide insight, but I believe the numbers that rejected evolution used to be much higher? As high as 50% at one point.
It does appear as though Creationism and ID are falling to the wayside. My suspicion is that the internet age coupled with the fallout from the Dover Trial probably drove several nails into the Creationism coffin.