Its all in how you ask the question
Unfortunately, Gallup requires a paid registration, so I can't get the exact wording of the question. But I suspect the finding that 68% want creation taught in the schools is not so much a bias as a result of a bad poll, but a true finding that people don't object if creation1 is mentioned in school. This isn't as alarming as it sounds. The evolution-creation controversy is, for better or worse, an important part of U.S. culture. Science classes should discuss and debunk creation "science" the same way they discuss and debunk eugenics, phrenology, or the myth that cats always land on their feet. Its also possible people in the first question of the poll are reacting to the (in my opinion) over-reaction of some educators who feel if the words "creationism" or "creation science" are even mentioned in school, the end democracy is nigh.
The finding in the subsequent poll questions, that a substantial minority actually favor replacing evolution with creation, is far, far more disturbing. Educators and administrators need to stop the knee-jerk censorship of religious ideas in the schools, and engage their students in a frank and open discussion of both so the students learn the difference.2
Notes
1. In my original post I apparently didn't proofread carefully, and this sentence originally read "...people don't object if evolution is mentioned in school." That sentence has now been corrected to what I intended to write. Sorry.
2. For example, I don't think science supports any of the other variations of creation "science", such as intelligent design. Read my contribution to Watching the Teleological Argument in action for more on this.