Here's a then-and-now photo gallery of the locations in Joshua Tree National Park used for the cover and inside album art for U2's The Joshua Tree in 1987 and 2007. That first juxtaposition is kind of heartbreaking:
(Personally, I suspect our own Jason Heller, who has a motive, of going after it with an axe.)
(Update: I just found this related page featuring an essay by the 2007 photographer, who offers a theory on how the tree died: "I don't believe The Tree fell by way of vandalism or human destruction. It simply collapsed from its sheer enormity. It was also an old tree, perhaps at least 200 years old. Joshua trees grow extremely slow because of the type of environment in which they live — dry and hot. I believe one of the limbs on the left side (see the photos with the band) snapped off, which caused a disproportionate amount of weight on the right side. The trunk then snapped because it couldn't counterbalance the weight. Nature, age, and physics fell The Tree.")
(First found here.)