Here is some more info on it (still trying to research the validity, but it seems legitimate):
The Legend: A marker beneath Sleeping Beauty's Castle shows the exact center of Disneyland.
Behind the Legend: The marker really does show the park's center, but it was not always beneath the castle. When Disneyland was first being laid out, a team of surveyors under the guidance of Walt Disney located the park's original center using a gigantic piece of string. The original center is at the middle of the large compass south of the castle. Over the years, as the park was added to, the exact geographical center has moved many times. But it was only after the construction of Mickey's Toontown that Disneyland executives decided the park had expanded enough to warrant a new marker. Using the ultra-high-resolution camera on one of Disney's "communications" satellites, a detailed survey of the park's new borders was made, and the new center calculated -- this time taking into account such technicalities as the non-Euclidian nature of surface geometry when measuring angles on the curved surface of the Earth. In a midnight ceremony shrouded in secrecy, the current marker was placed on the newly calculated spot.