The National Park Service was not prohibited from telling visitors the Grand Canyon's true age.
The press release that i recently highlighted here as an attack on science by a faith-based administration has been investigated and has turned out to be false. The press release quotes PEER Executive Director Jeff Ruch as saying:
In order to avoid offending religious fundamentalists, our National Park Service is under orders to suspend its belief in geology. It is disconcerting that the official position of a national park as to the geologic age of the Grand Canyon is ‘no comment.’
But Michael Shermer found that his claim could not be verified and is almost certainly false. See Skeptic magazine for details. I apologize for contributing to the spread of misinformation by passing this item along, and thank the anonymous commenter who pointed out my error.