The answer depends on the source you consult.
Harris Poll (Harris Interactive, July 6, 2005)
"While many in the scientific community may question why this issue has been raised again, a new national survey shows that almost two-thirds of U.S. adults (64%) agree with the basic tenet of creationism, that "human beings were created directly by God.""
US News & World Report
In Deputy Editor Robert Schlesinger's blog on US New & World Report, February, 11, 2009, Schlesinger analyzed the results of a recent Gallup poll that claims 25% of Americans reject evolution (that doesn't necessarily mean they believe in Intelligent Design, however; that information wasn't provided).
"I guess the upside is that the figure-39 percent-is a plurality of respondents in a new Gallup poll. Fully a quarter of Americans said that they reject evolution, while 36 percent say they don't have an opinion."
In the February 6, 2009, edition of US News & World Report, blogger Dan Gilgoff claims:
"During this last campaign, the topic of science-specifically, creationism and evolution-was pushed out onto the stage of the presidential debates. So much so that USA Today/Gallup released the results of a poll in which 66 percent of Americans stated that they believe in creationism."
Center for Science & Culture
On the other hand, the Center for Science & Culture, which supports Intelligent Design, claimed much higher percentages in 2001:
"Gallup poll after Gallup poll confirms that about 90 percent of the U.S. population believes that some sort of design is behind the world."
The Gallup poll site (see Related Links) posts the results of an August 2008 poll indicating 44% believe God created man in his present form; 36% appear to believe in some admixture of evolution and creationism/ID; 14% believe in straight evolution; 5% have no opinion.
Why is there so much variation in statistics? Because the results of a survey depend on the questions asked, the audience polled, the amount of bias involved in interpretation, the agenda of the person or entity reporting the information, and various people's willingness to manipulate data to support their case.
As everyone from 19th-century British Prime Minister Benjamin Disraeli to American Author Mark Twain have been quoted, "There are three kinds of lies: lies, damned lies, and statistics."