When I saw this during my first visit to Cima Dome in 2006, I thought it was an old sign whose message had been weathered away. It turns out to be a cross on the hill that is being covered to conceal the fact that it's a cross.
There's an ongoing debate about the appropriateness of having a religious symbol (the cross) in supposedly religious-neutral federal parkland, which is why the cross is temporarily concealed.
Sometime just prior to my current (2008) visit, the box over the cross had been removed to reveal the cross. However, my camera is broken and I can't document that. Christian vandals?