From Kurt at the National Park's Traveler comes this discussion of the Grand Canyon issue. This seems in rough agreement with Ranger X's comments reported and commented by me earlier. Also one of the commenters gives directions for finding the geological information on the NPS Canyon website. Indeed if you go here you do find a brief mention of the age of the rocks in the park...very brief. Kurt also has some insight as to the justification for allowing the Creationist text in the museum..after all they DO sell books that discuss Native American creation stories.
Clearly the issue is more complex than what PEER presented but why do I find myself a bit skeptical of all these explanations? PEER apparently stands by its statement.