Made into ceramic pellets the size and shape of marshmallows, the plutonium won't easily pulverize into breathable form that could cause cancer. Each 4-ounce pellet is wrapped in iridium. Two national labs have subjected RTGs to extreme heat pressure and pounding to ensure safety. And they've survived 23 space missions and three accidents without any significant release of plutonium. That's too big for a rocket to lift into space.
Nothing is riskless.
But Cassini's risks are so low as to be meaningless. And the knowledge Cassini can provide is immense. Saturn and its unique rings hold vital secrets about our solar system. The chemical fog of its moon Titan may even provide answers about the origins of this great blue planet Earth and life upon it. To be deterred from that journey by the false fears raised by Cassini's critics would truly be a giant leap backward for mankind.