Well, it has taken place already, at least the Pieta attacked with a sledgehammer. It was several years ago, but I remember it clearly. That was a Christian loony, not a Muslim loony.
http://www.newsoftheodd.com/article1024.html
I remember that too. In fact the thing has been accorded all sorts of high tech security now... I don't know if it is impervious to explosions, though.